Sounds like a case of the DW (Death Wobble) to me. The DW has alot of causes and fixes. The best thing I can do is point you in a good direction for reference as each case is different.
This forum is in its infancy compared to some others. As a good reference, go to JeepsUnlimited
and do a seach in each Grand Cherokee archive for both "death wobble" and "DW". You will find a wealth of information.
More than likely it is worn front suspension components (ball joints, trackbar ends, etc.) and/or a caster angle issue (you'll read plenty about this on the other site) from the lift.
I have a stock height 93 ZJ with the death wobble. A steering gear adjustment fixed 85-90% of the issue and I am trying to track down the remaining culprit(s).
With some patience, you will be able to find and fix the problem yourself.