First step....take a bottle of degreaser and your Jeep and go to the local do it your self car wash. Spray EVERYTHING underneath with a nice coat of desgreaser and blast it off. The valve cover gasket, oil filter adaptor o-rings, and rear main, ALL will eventually leak from the back of the oil pan/tranny bellhousing, mocking a rear main leak. This happens on all 4.0's....its just what they do. I've already done the valve cover gasket (very easy to do, but kinda expensive for a gasket), as well as the oil filter adaptor o-rings (which are much more complicated than they need to be on the newer XJ's) to mine.
The next morning (or later on that day when the engine cools off) stick your hand on the bottom of the oil filter and oil filter adaptor. If you get fresh oil, thats the source of your leak (VERY easy to change those o-rings on your year Jeep, as it's a different type of bolt holding the adaptor to the block). Also stick your hand behind the engine, if you get fresh (or somewhat fresh) oil, that would be the valve cover gasket that is leaking. If neither show any signs of oil, I would then move on to looking at the rear main.
One other possibility, which is one i've got right now, is the seal on your drain plug. They do wear out, and mine is currently leaking. It will drip a few drops over night, and every now and then coat everything underneath with oil....mainly after i've driven it on the highway for a while.
Do you do your own oil changes, or do you take it somewhere? If you do it yourself, what kind of oil are you using?
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