The following are the installation instructions for installing a two-speed axle in a 4104. I got these from Fred Edsel, A California BusNut who's "Phantom 4104" was the  Bus Conversions centerfold in 1994. The piece on Fred's coach mentioned the two-speed and he was kind enough to send me the following information.

I believe that this modification kit was manufactured on the west coast in the 60's or 70's. I don't know who the author is and I take no credit (or blame) for the contents. I have retyped the instructions and did some clean up on the drawings as they were about a tenth generation copy and barely readable.

If anyone knows who the originator is please let me know.

I am unable to do anything with this information but I hope there are people out there who are knowledgeable in rear ends to decide if this is worth investigating and if the parts are still available.

Apparently this includes parts that were fabricated by the originator. I hope some expert out there can figure it out.

The following is the text of the instructions. Words in Italics are those which I could not read and offer my best guess.

TWO-SPEED BOLT ON TRANSAXLE
The auto and truck industry has been acquainted for many years with auxiliary transmissions of many configurations. One type of application for light and medium trucks is built inside the differential case using a planetary gear set. Eaton Rockwell manufactures these units for applications as high as 35000 GVW range. Another style made by Timkin is a double reduction two-speed using two ring gears and two pinions with a shift between them giving two choices of final drive ratios. These factory offerings would be quite economical for 4104style drive lines if the '04 shaft did not project at 60 degrees rather than the standard 90 degrees as is used by most vehicles. Shifting control has gone from direct linkage to vacuum, electric, air, and hydraulic as the manufacturer elected.

The challenge of offering the foregoing accessory in a reliable form to owners of converted 3751 and 4101 series GMC coaches has been both frustrating and rewarding. Parts were to be standard as much as possible and available from ant large truck wholesale house. The ending assembly needed to be a sturdy, dependable, simple bolt on design with a reasonable and practical gear ratio.

The parts were selected from a Spicer 2300 series remote air shift transmission. The gear ratio available is 1.3 to 1 underdrive or .7 to 1 overdrive and direct, depending on the manner of assembly. Shifting is powered by regulated air pressure controlled by electric solenoids switched by the driver of the vehicle. Standard parts are listed as noted, with the exception of the case, stub shaft and clutch gear which are special. The input drive gear is welded to the input stub shaft and may be replace by cutting the weld with a lathe, then rewelding shaft to a new gear - similar to rewelding drive shaft inserts. A stock of clutch gears is on hand in Oceanside California. 

The assembly contains the air shift cylinder which must be taken apart. It is sealed with "O" rings. Care must be used to avoid damage or loss. This cylinder and piston is reassembled after the transmission is in place and the shift yoke can be freely moved by hand. The air and electric connections are rather self-explanatory.

All bolts and retaining caps etc. are shipped hand tight without ant sealant and must be sealed and tightened as good practice dictates.

Installation is somewhat in the following order:

Raise the coach on blocks (12 to 14") or place over service pit; block wheels to prevent rolling.

1.Remove "U" joint from axle end of drive shaft.
2.Suspend drive shaft with rope; pull up high oput of the way.
3.Remove "U" joint flange from rear axle pinion.
4.Drive shaft may now be removed from vehicle.
5.Remove carrier bearing housing. (Be sure to save and secure shim pack for re-use in assembly to maintain tooth pattern). Pinion and bearing will remain in axle. The outer taper pinion bearing may be frozen or rusted to the shaft preventing the housing from moving out of the differential case more than approximately ¼" A Porta-power type puller may be used to pull the pinion cage and push the shaft back into the housing. A word of caution - if more than 6 tons of pressure is required to remove the bearing, the entire differential must be removed and disassembled in a shop to avoid breaking the jacking bolt flange area. If this breakage occurs, a replacement housing is required with complete re-setting of the tooth pattern on the ring and pinion gear.
6.Cut seal housing from bearing carrier flush with outer bearing cup.
7.Cut bearing housing register size to fit adapter plate. Some lathe time can be saved by sawing through the seal oil return "boss", then mount housing in lathe arbor to cut off seal area and trim flange and register to size of adapter plate.
8.The pinion has been originally set and shimmed for proper tooth pattern, with the installed bearings; therefore to establish shaft clearance in the new case before bolt up, using original 641 bearing, stack thrust washer and clutch gear with plate and bearing carrier to determine the depth position of drive gear.
9.Shim to .040 between drive gear and clutch gear. See Diagram.
10.Install pinion bearing carrier spacer shims and adapter plate. Use silicon sealer between shims and between metal surfaces.
11.Reinstall crush rings and outer taper bearing, 4.25" thrust washer, low gear, clutch gear and needle bearing.
12.Torque self-locking nut to 800 ft. lbs. and tack weld in one spot. (Removal by disk grinder)
13.Shorten drive shaft by 8.75" and reinstall at transmission and suspend axle end high under vehicle out of the way. Be sure yoke or spline is welded exactly in the same rotation position to avoid placing "U" joints out of time with each other. 
14.Install shift clutch and rail fork assembly. Care must be used when installing sliding fork and sliding clutch ring. The clutch gear and ring are heat treated after machining, as a result some slight distortion occurs - sometimes as much as .0005 to .001 out of round tooth diameter. This makes for stiff shifting or dragging of the clutch slide. These two points are marked with paint to assure the most free movement. Rotation of one tooth often makes considerable difference. The ring is marked "front" or pointed to the "U": joint. After the gears are used for a few miles the tooth roughness wears away.
15.Install counter shaft bearing in housing. (Items 15 and 16 are assembled before shipping)
16.Assemble driving gear and bearings in case without "U" joint yoke. Use spacer sleeve and nut to hold in place. Install counter shaft secure with thrust washer.
17.Place one-inch round bar stock across upper radius arms for hoist.
18.Use cable or chain come-along to lift case into position.
19.Wrap cable around case. Use slip wrap with clevis, suspend from bar placed over radius arms.
20.Coat surfaces with silicone.
21.Hoist outer case into position while rolling lip over shift bar, when bore of shift bar is near position, guide case onto bar till counter shaft touches lower bearing, guide counter shaft race into bearing. Case should now close - metal-to-metal.
22.Bolts form dowel pins. After all are in place - tighten evenly.
23.Install yoke, key nut to shaft at proper bearing pre-load.
24.Assemble "U" joint and connect drive shaft.

PARTS AND PACKING LIST

85-30-9		Counter Shaft (Field Modified)
85-8-23		Gear
70-196-29	Gear
70-8-55		Gear
500010-19		Key
85-465-6		Clutch (Shift)
85-466-7		Clutch
85-381-10		Lock Ring
85-381-7		Lock Ring
82-66-22		Fork
82-67-18		Rail (Field Modified)
550519		Bearing (Needle)
MSN-1311-GELR	Bearing (Roller Thrust)
MR-1311-EAHL	Bearing (Roller)
633			Cup
641			Cone (Taper Roller)
82-22-1		Cover
330665		Plate (Retainer)
28P19			Seal
780D			Set Screw
2317696		Seal    GMC
6-4-4421		Yoke
2221248		Lock Nut   GMC
L46526102		Shift Valve   Schrader Bellow Div
2232427		Shims   GMC .010
2243609		Spacer (2)
			Spacer Thrust Washer (Spec)
			Housing (Spec)
			Adapter Plate (Spec)
			Pneumatic Cylinder (Spec)
			Stub Shaft (Spec)
			Clutch Gear (Spec)
			Regulator - Air (Standard Part)
			
OTHER MISC AIR FITTINGS AND HARDWARE