I don't know about the ZJ, but an XJ to a TJ the engines shouldn't be a problem. to my knowledge the tranny and x-cases will be. I've not done it but a book I have (wrangler tj builder's guide) states you'll be faced with major mods. you're better off finding a tj getting parted out for the tranny and x-cases.
the same book recommends that if you are going from a 2.5 to the 4.0, you might be better off going to a small block chevy v8. the book says the swaps are ~ the same price and labor, for much better performance. (that being said, nothing beats a 4.0 for durability, IMO.) the book also suggests this web site:
Novak Jeep Conversions - Home
which I went to, and wow....the free knowledge base is amazing. check it out.