change 23 spline to 21 np231

My T-case is shot. I have an extra np231 (23 spline) that I want to swap for my 21 spline (87 XJ). Can I just take the old 21 spline input shaft and place it into the new 23spline case? How hard is this to do or will it bolt right in?
Thanks!

Depends. there were some changes to the planitarys in the 231. What year's are the two cases?

The pitch on the teeth in the planetary gear changed in 1995 so you can't mix pre and post 95' input gears. If the 23 spline t-case is pre 1995, you can swap the gears. You will need to completely disassemble both t-cases then swap gears. It's not hard but can be intimidating if you've never taken a tcase or tranny apart. A factory service manual would be great here. You will need a good set of snap ring pliers and I highly reccommend replacing the input gear seal and front output seal while you have it apart.

take a picture of the open case before disassembly