1998 Jeep Cherokee Fast ANOYING Vibration >55mph

I just bought an 1998 Cherokee with 160,000 miles on it. It has been female driven and Dealer maintained for its long life. Looks good inside and out, even the oil at 3,000 miles is pretty clean. There is one problem I cannot figure out, nor can the mechanics I've taken it to. This ridiculous vibration above 55 mph! The tires and wheels have been checked. The rear-end was badly in need of a bearing kit, I had a local transmission specialist rebuild it and put two new wheel bearings at the end of the axel shafts. Drive shaft and U-joints checked out (straight shaft - no bearing there), the front end components all look pretty new and are tight. I am getting ready to replace the drums and shoes in the back but seriously doubt that could be it. The front calipers look good and rotors / pads look fairly new too.

The vibration starts around 55mph and gets progressively worse the faster you go, it seems to be at a much higher rotation than the tires are moving which leads me to believe it is in the drive-train, but where? It feels as though a massager has been attached to the body of the vehicle, seems to be everywhere! I cannot pinpoint a place of origin. I drive back roads to avoid highway speeds because it is driving me crazy! Any advice would be greatly appreciated and could save me from the men in white!

Thanks.