If your T-Case output shaft is bent it could also cause this sort of vibration. I know its rare, but Id say take the drive line off and watch for any sort of out of line of the shaft, I ran pretty sharp angels on my Jeep YJ with 6lift and never had vibration problems till I blew out a u-joint. Had a vibration at higher speeds and could never find it. Sold the jeep and about three months later I talked to the guy that bought it and he said that he had a slip yoke eliminator kit put on. The shop that did the install told him the output shaft was bent. I got a Slip Yoke eliminator kit and drive line for my XJ from Tom Woods for about 460.00, worth every penny.