Snapping Noise at high Speed

[thisNthatcar]

I purchased a 96 GC 4.0L Automatic, Selc-trac with 86,000 miles. Vehicle runs great shifts smooth I have used the 4X4 during several snow storms and it pulls great shifts smooth, I hear an increase in noise in 4X4 but not abnormal. I have been driving the Jeep to and commuting 15 miles each way work for a couple of months on secondary roads usually at speeds from 40 to 60 mph. The vehicle runs string with no signs of a problem.

Now, I recently took the jeep on the highway cruising 70 – 80mph in 2wd of course. About 20 miles into my 35 mile trip I heard a snapping noise from the rear, a few minutes latter from the middle and a few minutes after that from the front. When the noise happened in the front I felt a small vibration in the steering wheel. The noise is not that loud it almost sounds like hard plastic breaking. During the 70+ mile trip this happened 3 times.

Dealer check it out and reported nothing wrong. However they only drove the Jeep 5 miles...

Any ideas what happening?
What should I begin checking?
FYI: All tires same size and equal air pressure

Thanks