I just replaced the rear main in mine. At 26,000 it was leaking like a sive. Got replaced at 19,000 when the trans was done too.
I am pretty anal about cleaning my Jeep after its been wheeled (ask penguin_in_a_cj) how long we are under my jeep spraying it down with a pressure washer the night we get home and the next day.... but dirt/sand/mud in or around the rear main will chew up the seal very fast. If your playing in sand or mud, you deffinatly want to spray out the bell housing. You can take a pressure washer and just spray inside the two vent holes (thats what I did, and 0 mud/debris inside my bell housing) or you can just remove the 4 screws or whatever it is, and spray it real good that way. You wont be able to actually get to the rear main, but getting the area clean will help more than anything.