If it has come from a 96 4.0 then it should be a Dana 35 with LSD and ABS. Your 98 4.0 will have the same but no LSD.
The earlier ones with LSDs have more wear issues than the later ones due to the wear of the LSD clutch plates leading to contamination of the diff oil. This is why I suggest changing the oil when you get it - it also gives you a chance to look inside for obvious wear. With a LSD the additive from Jeep is vital too.
Brakes will be the same and swap over no problem - keep your brakelines and handbrake cables cos undoing them can be tricky and makes more work for you.
You'll be able to tell if it's a Chrysler 8.25 axle cos your u-bolts won't fit around it, it weighs a lot more, has no ABS and has a flat 'lip' along the bottom of the diff housing.
If it's a Dana 44 for an XJ, sell it to me for a pork pie and a packet of Jaffa cakes