my stock 97 xj will ride smooth until I begin to accelerate past 55mph. I feel it in the body of the jeep more than the steering wheel. the alignment, and rotation/balancing were just done. shop said my suspension components were in good shape. tires are less than a year old. the vibration seems to go away for the most part accelerating into 65-75 mph speed range. both hub assemblies, brakes/ rotors are fresh as well

 

Well it could literally be almost anything... but it sounds like tires need rebalanced. Take it back have them take another stab at it. Or rotate them around see if there's any changes.