99 Suburban has severe front end shake only when towing.

Possibly a out of balance tire or one that is separating internally. Having them balanced will tell if this is the issue or not.

Thank you, The tires have been balance twice to make sure that wasn't the issue. Internal separation is something I hadn't thought of. I had planned on replacing the tires any way. We discovered the tires came off another truck that had been sitting in a salvage yard for quite some time, and are try rotted between every thread. Thank you very much for your input. Great appreciated. I'll respond back later this week, once I replace the tires.

Ok. With dry rot in the treads I would bet on internal separation. Look at your wheels. Does one of them have a lot of wheel weights on it. If so they had to add all of these to balance it because of internal separation of the tire. Went and had mine balanced the other day and they had to put 10+ ounces on to balance it. Future investigation showed that the tire was separating on the inside. It also had small dry rot lines in the treads.

So what can it be when the truck goes to 45 mph it shakes real bad after 45mph it stops shaking what can it be