Found a really good write up on replacing the rear main seal and oil pan gasket. I'm going to put this here so I can use it as a reference when i do mine soon.
Be sure it a rear main. 4.0's are notorious for valve cover gasket leaks at the rear corners. It will look like a rear main. Some have enough trans gap to leak down around the rear main and out the bell housing, so just be sure you clean it up real good and pinpoint your leak. They can be sneaky!
on a side note, while I had my valve cover off, I tried to pry out the PCV valve near the front of the motor and broke a corner of the flange off...not its leaking, I know those elbows are replaceable on most older motors, but it seems to be a permanent fixture on mine? anybody ever replaced those? Now I need to replace it because i'm getting some oil seepage around the broken fitting.