i was thinking about the pinion bearing. it will be more noticable at high speed and the sound goes in and out of phase with the tire noise. for a quick check go under the car and push and pull really hard on the drive shafts with the jeep in nutral and the parking brake set. there should be NO movement, and you should eb able to do pull ups on them. if it does move find the source of the movement. roll the shaft back and forth (it will let you a little) the caps on the u joint should only rotate, if you see them move up or down or in and out its a bad joint.