Rear main is easy to change. My Cherokee had an 8 inch lift with 35's, locked f/r, 4.88:1. My last run back from the Hammers had me at 19 mpg with all of my spares, heavy bumpers, skid plates, etc...that was with the cruise set to 80.
Like others have said, i recommend the HO motor (91 and newer) with the AW4 auto. Try to get one with the 231 transfer case, the 8.25 rear end (D44 is tough to find), and a high pinion Dana 30.
Feel free to ask any questions...sounds like a few of us know a lot about them.
Thank You!
OldMoparsRule!