I also have an XJ with a 242. Mine is an '89. I put a 3" Skyjacker lift in it 18 months ago. Imediately after the lift, I installed 31"x9.50's on stock wheels, then went for an alignment. I imediately noticed vibrations in the 30-40 mph range and ordered a transfer case lowering kit. Once that was installed, vibes mostly disapeared...but not entirely.
I took it to a reputable shop requesting an SYE kit and CV driveline. He measured my rear driveline angles and recommended against...said angles were good and that vibration was likely "transmission shutter". That was over a year ago...vibes still present at 30-40 mph speed, but nothing else...no other damage.
I still want to do an SYE and CV driveline...and will, but no rush that I can think of.
I'm guessing that a t-case lowering kit would do wonders for you. Also, I should mention that my lift came with new rear springs...and they changed the angle of the axle. Your lift may not have done the same. You should check driveline angles fo the rear.