As has been pointed out, the pre '96 R150F and R151F input shafts are far too short to be used in the AX15 bellhousings. (Even with the Advanced Adapters pilot bushing) The R154 input shaft is longer, and, with the proper spacing and AA bushing CAN be used in the AX15 bellhousings with no mods.
Some further information from Marlin "Crawler" Czajkowski on the Toyota R series and the AX15. It is possible to swap the longer AX15 input shaft into the pre '96 R150F.
This info applies ONLY to the AX15 and R150F.
"Parts are interchangeable between these trannys. Inputs, output shafts, front nose cone, and rear housing are different but generally can be interchanged if you stay with in the family.
There are three "R" families;
86-91 early, simple, common, heavy design
92-95 upgraded syncrows, hubs, shift rods and linkage, smaller bearings, lighter gears
96-newer greater helix angles, upgraded syncrows, lighter bolts, reverse brake
The Jeep 86-00' AX15 would use the same bearings, syncrows, and gears within the same family as the Toyota boxes. I am not sure of the Isuzu production years, but would also be the same.
Remember that only the 86-87' 4x4 R151F 22R Turbo got the 4.31 1st gear. 2x4 22R Turbos, got the 3.83 1st, along with all other 4x4 R transmissions --- Toyota 3.0, 3.4 V6, Jeeps, and Isuzu's.
The lowest geared R is the 5.15 1st gear found in the R452 imported trans which also uses the same bearings, and syncrows.
The Jeep AX15 has a 10 spline American style input with a 5/8" pilot. I am currently converting a Jeep AX5 to a Toyota t-case
I have never had to opportunity to upgrade an AX15. This can be done but only to a 86-91 AX15. The longer coarse spline input and Jeep nose cone will swap right in. The AX15 bellhousing will also bolt up to the front of the R151F since both front housings are the same!
I send both Toyota and Jeep shafts to my drive line shop and have them put together."
Also, the question was asked (on another board) if the AX15 could be hooked up to Toyota transfer cases for crawling. According to Marlin, that's a go..
"I have put a Marlin dual case behind a AX5 [G54/58 Toyota] Jeep trans by changing the output shaft and tailhousing from a Toyota doner trans.
This can also be done to a AX15 [R150 Toyota].
The Jeep auto should also work as long as the output shaft and rear housing is changed from a Toyota trans.
In the past years, I used to get a lot of calls from Jeep owners wanting to go slow like the Toy trucks. Since I was so busy, I directed the calls to Mepco and Klune V."
And... as to WHY a longer input shaft can't just be swapped into an R151F:
"The problem is that the 4.31 and 5.15 trans were only available with the short, pre 96 input shaft. It is impossible to just replace the input gear only because it is so much smaller. It is so much smaller in diameter, that it would not even touch the front constant run teeth on the factory countershaft. To obtain the 5.15 ratio, the R452 uses the turbo first gear set, but a greater countershaft underdrive by decreasing the input gear diameter, and increasing the countershaft diameter. This combination will effect 1,2,3,5, and reverse ratios.
Marlin" http://www.marlincrawler.com/
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As an example of what can be accomplished:
Pic 1: Bellhousing from a 3.0L Supra Turbo attached to an 89 Jeep AX15. The input shaft is 7.82" in length but 6.55" to the edge of the threads.
Pic 2: Toyota Supra Turbo R154 next to Jeep AX15. Input shaft is 7.25" in length but 6.65" to the edge of the threads.
Pic 3: Supra Turbo R154 next to Jeep AX15.
It is obviously possible to swap the 91 and earlier Jeep input shaft into an R150F and use the Supra Turbo bellhousing to hook it up to the 7MGTE. However, this shaft swapping can be avoided if using the 1996 and later R150F which has a longer input shaft already.
Pic 4: Shows the relative shifter locations between the various 2WD transmissions.
Greg