Hi All, noob to this forum, and I have a BIG Tahoe Problems

I've owned my 96 Tahoe since late 97. 4X4, 4 door, 90,000 miles, 4" SuperLift, BFG 33X10.5 AT tires. On a trip last weekend, the thing started vibrating like mad. Here's a quick list:

up to 59 MPH OK
60 to 69 MPH BAD vibration. Things are blurry in the rear view mirror. A sound and feel like you're driving on the rumble strips on the side of the interstate.
70 and above Slight vibration.

Since it was a long drive, I had time to mess with speed/RPM. In the 60 to 69 MPH range, slight pressure on the gas seemed to lower it a bit, but it was still bad. Going downhill, at the same speeds, off the gas 100% and once the tach dropped below about 1700, it stopped. Smooth as silk.

This happened all of a sudden. Three weeks ago and another long drive, no problems. I've checked the u-joints and they are tight. No drive shaft play. This happens in OD or when it down shifts for a uphill pull.

Has anyone experienced this? Suggestions as to what to check?

Any and all replies will be appreciated!