AX-15 Interchange...
1989.5-1990 will have the "small" pilot diameter (.590", IIRC - same as the BA-10.) Will require a pilot bushing change.
1991 (I think) on up have the larger pilot - somewhere around 7/8". Can be swapped into earlier rigs with a pilot bushing swap.
The hydraulics break happened around 1993/1994 (internal slave to external,) and swapping one way or the other requires a full hydraulics swap. Note that (for Peugeot tossers...) the Peugeot hydraulics and friction will work with the AX-15 "internal" version without difficulty.
Spline count? All AX-15 gearboxes are 23-spline output, all Peugeot gearboxes are 21-spline output, and all BA-10 and AX-15 use a 1-1/8"-10 input shaft. Same length as well - so they swap readily.
Note that the "internal" AX-15 can be converted to an "external" AX-15 by swapping the bellhousing, fork, and front bearing retainer. Also note that the AX-15 can also be had from later Dakota pickups, and can be "converted" there as well by swapping the bellhousing and associated parts. The bellhousing for the "external" AX-15 will also work with the NV3550, should you find one (the NV3550 cannot be converted to an "internal" setup - but why would you want to?)