Is there any way to modify a wj rear axle to fit on a xj ?

of course, if you can weld you can swap anything under. the WJ axle is wider then an XJ axle so you'll have to do something about the front axle too.

It’s 5 inches wider & 5 on 5 bolt pattern. If you already have it or getting it for free, sail it & get a 95 up Explorer or Mountaineer 8.8. It has the correct bolt pattern & it has 31 spline shafts.
I can attest to it being a strong axle. I beat the ever living snot out of mine & it gave me zero issues.

Why would you want to do that? Different bolt patterns and it will either be a Dana 35 or Dana 44a, both of which are weaker than a Chrysler 8.25.