I have been chasing driveline vibrations for 2 years on the cherokee. Finally decided to do SYE this spring. Driveline angles were set properly with shims on the axle. I still had vibrations. Then, I decided to bring the double cardan driveshaft to fleetpride to have it rebuilt and balanced. They did all of thta. Still had problems. I noticed that if I would drive for extended periods of time, they would "creep" down slower and slower. IE - on the way into work in the morning, I could go 55 with no problems. But on an extended highway run, I would have problems with 45 by the time I got to my destination. I decided to dump in some 85w-140 oil to see if that helped. Oddly enough, It seemed to help a bit. I can now run 65/70 with minor vibrations. Last night I drove it at 70 for probably 15 or so miles and the vibrations didn't get to the point I thought I was going to get an inner ear infection or have something let go. Guess my rear end is toast. Apparently something is so worn out that the 80-90 would get too "thin".