There are several parts to replace, the push rods, the rocker arms, there is a cradle another piece, I don't recall the name, look on rockauto.com, they have a rocker kit, you get everything you need to do one cylinder, it isn't a hard job at all but if you can find a clymer or haines manual so you have all the specs right.
Now I want to agree with the previous poster, 9 times in 10 it is a manifold issue, even the 4.0s are notorious for it, every 2.5 I've seen as well as mine have a little clatter st times that make it sound like a diesel, mine does it at about the 10hr point in long trails or drives. Yours though I'm betting is rockers or a bad lifter if you can control it. Good luck and check out that website, Jerry Bransford first clued me in to it, can't beat the prices.