I think ou are asking about changing the rear mail engine seal.. If so the first thig to do is check all of the BREATHING hoses on top of the engine and the PVC valve. If any of these is not operating correctly you will build up pressure in the engine and the first place it comes out (with oil) is the rear main seal.

Now that said.. to give you a hint of just where it is. When Mopar builds an XJ the first thing they do is to place a rear main seal on the start of the assembly line! Then as the line moves an XJ is built around that damn seal!
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />