I have not worked on the diff yet. I do have most of the parts to change my gears to 4.56 and install OX lockers. I have read a lot of articles but do not have any first hand knowledge yet so take my comments with a grain of salt.
You should be able to replace the rear pinion seal by removing the drive shaft and yoke and then pulling the seal and put in a new one. There is a crush sleeve that is used to set the pinion bearing preload. Some people say that you must replace this every time the pinion nut is removed. Make sure you pick one up when you get your new seal. Here is a site that has lots of information about differentials.
Ring and Pinion
Note: If I am wrong here several people will pipe in to let me know
2000 Jeep Grand Cherokee V8 Stock
93 YJ I6 hard top (really belongs to SPEED)
RE 4.5" Extreme Duty Lift, Advanced Adapters SYE, Tom Woods CV Drive Shaft, EA589 10" rims with 33" x 12.5" MT/Rs
Bullet Proof Front and Rear Bumpers and Tow bar