I own a 2006 Cadillac ESV platinum edition and we often use it to tow our 21 foot Ranger Commanche to Canada. The truck tows fine until we reach speeds above 60 miles an hour or so. Once that speed is reached, the vehicle begins to vibrate pretty severely. Especially the steering wheel. After getting out of the truck your hands are almost shaking!! Without the trailer at that speed there is no problem. We have also tried it with other boats with the same effect. We had the tires balanced and rotated which didn't help. Does anyone have any suggestions as to what the problem could be?