"Transmissions used in later AMC vehicles came with either a 21-spline or a 23-spline output shaft. Transmissions coupled to four-cylinder engines normally used 21-spline output shafts. The 23-spline manual transmission was universal in the Eagle lines and was generally used with six-cylinder applications. All transfer cases were available with inputs matching either 21- or 23-spline shafts, so transmission swaps are possible among various models and years.

The Aisin-Warner four-speed automatic transmission (AW4) was co-designed by AMC and Borg Warner then built by Aisin in their new facility for use in then all new Cherokee XJ's new inline six-cylinder 4.0 L design. It was manufactured by Aisin-Warner, a member of the Toyota group, in partnership with Borg-Warner. It shares many parts with the Aisin 450-43LE that is used in Toyota off-road vehicles.

The AW4 is also used behind the 2.5 L (150 in³) AMC Straight-4 engine. It has a removable bell housing with a mount for the crankshaft position sensor. Early AW4s used 21-spline output shafts. In 1991 the AW4 was changed to a 23-spline output shaft, concurrent with adoption of the High Output (commonly known as H.O.) inline six-cylinder engine. Some sources state that the output shaft spline change occurred for the 1990 model year. The change most likely occurred during the 1990 model run with early 1990 models receiving 21-spline transmission and later models receiving 23-spline. If swapping transmissions in 1990 model year vehicles either swap the transfer case as well or make sure to check spline count. The input on the transfer case can also be changed to match the output shaft spline count. Rear-wheel-drive-only transmissions all have the same spline count on the output shaft, only 4x4 models are affected by differing spline count.

Gear Ratios for the AW4:

1st: 2.80
2nd: 1.53
3rd: 1.00
4th: 0.75 (23-spline, 0.705 21-spline)
Models that used the AW4:

1987–2001 Jeep Cherokee (XJ) 4.0 L
1993–1993.5 Jeep Grand Cherokee (ZJ) 4.0 L
1987–1993 Jeep Comanche 2.5 L
1987–1992 Jeep Comanche 4.0 L "

So it seems the Jeep AW4 Autos were available in either spline size depending on the engine size. You would just need to specify a 21 spline Atlas. HOWEVER, The rear case adapter and output shaft will not be the same length as the Vit. The mounting bolt pattern will also be in SAE dimensions, not metric. See pic below.

As the AW4 is virtually the same as a Toyota A340 Auto used in 4Runners, you could also do a similar swap to the Manual one I showed above for a shorter overall length, but the down side of this is the Toyo Auto is only made for Chain-driven transfercases with limited (ie: NIL) aftermarket reduction gear choices. You will need to get an adaptor from Inchworm to fit Toyo gear-driven cases, or even DUAL transfers!

I have two hilux gear driven hilux cases here, I was planning on going duals, but by the time you add up the cost to build them strong and properly, in my opinion you may as well add the extra coin (which isn't all that much)and buy an atlas, that will be alot stronger then a lux case will ever be?

This is what I would do if I had my buggy build over again.