I'm going to say wandering tread on the tires. Had a Tahoe that would wobble, and over 80MPH it got to be downright dangerous. Had it on a dyno, noticed that as the speed got up the tread on the tires actually started twisting from side to side. When a piece of chalk was mounted to a stick so that it would leave a mark on the tire while the tire was rotated showed that while the chalk line was straight, the tread shifted back and forth under the chalk. Normally this is used to see if the axle is bent, but in this case it was the tire itself that had bad tread. A new set of tires corrected the problem. A better check would be to find a tire shop that has a Hunter load force tire machine, and have them rebalance the tires using it. It will pick up if the tire tread is shifting.