[denverpete]

I've been following this thread and Larez2's comments finally reminded me of something that happened to me (not in the Box).

I had a Jeep Wrangler that started to vibrate at highway speeds. I took it in and had the tires balanced. Went out on the road and still had the vibrations. I checked the wheels and found MASSIVE balancing weights. Hmmm, first clue.

Turns out, one of the tires had a "belt" separation or was dented inside the tire. Replaced the tires with nice, new, bigger ones and the problem was GONE!

Seriously, I think this is a tire issue. If you have a friend with a similar Box - maybe you can swap out the tires and see if that fixes the problem. If it's a belt separation (or dented belt) inside the tire - no amount of balancing is going to solve the problem....