Ever worked on a ZJ or an XJ?
Its much easier to replace the t-case on a ZJ then it is to cram a V8 into an XJ. I also know lots of people who wheel just fine with the AWD tcase and do fine.
Let me put it this way, its easier to put a V8 in a TJ then it is an XJ.
I'll stick to giving advice on Cherokees and will leave the TJs to those who have actually worked on them.
Anyway back to the original poster, HP diffs are over rated. If you get a good deal and have to swap it anyway then go for it and swap it in. Just remeber to check the gear ratios. ZJs came with either 3.55s(most 4.0s and some V8s without the tow package) or 3.73s(tow package and upcountry packages). XJs came with 3.07s(4.0l and 5psd) or 3.55s(4.0L and auto) or 4.10s(4cyl and 5spd).
So if you have a ZJ with 3.73s then an XJ diff won't work for you without a gear change.
BTW I have a ZJ 3.73 diff that I could part with....