At 6" of lift, a T-case drop might not be enough. U-joints have a limited amount of angle that they can operate smoothly through.
You likely will need a Double Cardan drive shaft. Someone else here posted about finding one for use on a late model XJ that did NOT re-locate the slip yoke or require a SYE (the later T-cases apparently do not leak fluid if you drive with the rear shaft removed, a big oversight on the earlier T-cases).
Also, don't ignore the front shaft. If your CASTER angle is close to spec, your front PINION angle is likely off enough to contribute to the problem, and a T-case drop will make this worse.