Ok so this may be a stupid question but is there a difference between a NP 231 AC (out of a 88 XJ pioneer) and a NP 231 J (97 XJ Sport)? All I can figure is the gearing may be different?

 

Not stupid at all. There is a difference,count the splines on the 88 it should be 21,the 97 is 23. Ax-15 transmissions have 23 splines and aw4s were switched to 23 spline when the h.o motors came out in 91.
Count the splines on the 88,should be 21.

 

the shafts are changeable if you need to swap transfercases...