Do a search on rear main seal, it should come up with some threads that describe the process. You've got it easy with a 2 wheel drive, the trans comes out fairly easy. I cheated on my 4x4 and just slid the transmission and xfer case back about 4"-5"(didn't even need to remove the crossmember)and was able to get to the seal carrier. Just be sure to install the seal
dry or it will leak. When I did mine because it's so hard to get in there I replaced my flexplate at the same time, just so I knew it was fresh as I already tore the center out of the original plate. I got the seal and flexplate from Cummins and the total bill was just over $100 U. S.
Hope this helps,
Caleb