[fredjacksonsan]

The 99 and 00 4.0 engines were built with a defective head that cracks and starts to leak at about 80 or 90k miles. It seems like a head gasket defect, and you can get coolant in the oil or loss of coolant through the cylinders. This only shows up after the vehicle gets really hot; if you just go on a short test drive you might not see it.

The bad heads have "0331" stamped on the side of the head, underneath where the intake and exhaust manifolds are. The solution is to get a pre-99 head from a junkyard and have it installed, junking the 0331.

Other that that I don't know of any issues with particular years...but since the cooling systems on these degrade without maintenance, you'll want to have a good look inside the radiator for corrosion, and keep an eye on the temp gauge for awhile.