Depending on the Tranny and the type of Tcase your 89 has, it may either be a 21 spline input or a 23 pline input. chances are, its a 21. Your 96 is with out a doubt going to have a 23 spline transfer case. Also, chances are theyre both NP231J's. So swapping them would be pointless unless one is damaged.
Suspension, on the other hand is straight forward, all XJ's used the same suspension, from the time they were introduced until the time they were killed. Nothing really ever changed.