i have an 85 xj 4dr 4x4 with the 2.8 v6 automatic and the 207 tcase. i would like to upgrade my stock np207 to an np231 so i can access more aftermarket parts ie sye and 4to1 kit. will the 231 be a direct swap in my xj? if not, what are my options with the least amount of modification?  i know i will have to make drive shafts mods, but what else am i looking at?  

It will bolt right up. You may need to modify the cross member mount.

Depending on the 231, the input gear may have to be swapped. The older 231s and the one that you have, have 21 spline input gears. Newer 231s have 23 splines. There is also a long and short input gear. Pre '91 either YJ or XJ is your best chance at getting the right input gear. If you don't want to switch the input gear make sure that what your buying has the right one. It's not a big deal of a swap but it takes time and costs money.

It is difficult to know for sure what tc input gear any Jeep has until you look. Only guidelines.  

Click here for more info on input gears.  

thanks for the help.  i all ready have a 231 out of an 88 xj with the 21 spline input. i just wanted to be sure it would work before i tore it down for a rebuild and spent the money on a sye.