Danno -
Check out which axle you have in your rig currently. XJs came with 4 different rear axles (more, actually, but only 4 are meaningfully different)
1. Dana 35. Sucks. Don't put any money in to it, don't plan on doing much with it. It just sucks.
2. Chrysler 8.25, 27 spline (out of a 96 or older XJ with no ABS) This is a decent enough axle for light use. Some came with posi rear, almost all with 3.55 gears.
3. Chrysler 8.25, 29 spline (97+ with no abs) This is a fine axle for most use. I think CRASH rated it for up to 33's.
4. XJ Dana 44. Hard to find but pretty sweet. Will hold up to 35s, generally speaking.
All of the above can be locked, geared, given disc brakes and perhaps most importantly, found fairly cheap.
You could also check craigslist and your chapter's for sale page for axles that have previously been bolted in to an XJ, like a Ford 8.8. If somebody else did the work already, what does it matter if it would have bolted up from the factory?