I just replaced my rear main seal on my '99 4.0L. I found a few different how-to's just by googling "jeep rear main seal". They were all very useful. I recommend this job to anyone. Greasy and dirty but if I can do it, anyone can. I did it alone in about six hours. Good luck. I am happily not leaking any oil for the last two weeks!!!
http://www.jeepin.com/features/rearmain/