You should be able to take the input gear out of yours and install it in the NP231j.
AMC didn't have any t-cases of their own that I know of. If its a 231, its a New Process t-case. It might be a "c" style but I kinda doubt it. Since that has the front driveshaft coming out of the passenger side. If its old enough it might have a NP207 in it.
'87 YJ, 4.2L built and bored, 8.5" of lift, 8.8 rear, AX15/NP 231, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...