vibration/ frame beaming?

[Bad Vibrations]

I don't know if this is the right place to put this message but I have to start somewhere. I have a 2003 GMC 2500 Crew Cab 2wd,6.0 bone stock except for the purchase of new tires. That is where my problems with a vibration started. The stock tires were Firestone Steeltex A/T 245/75/16. The truck rode like a cadilac(for a 3/4 truck) The truck has about 47,000 miles on it and I wanted to get new tires for the winter. A guy at my work has an 06 4x4 just like mine and he put 285's on when he bought the truck. Says he has no problem just bumpped up his torsion bars and everything is great. So I go in and get 285's on mine and have them bump up my torsion bars to level out. The first time I hit low 40's mph I get a vibration that feels like one of the rear tires is out of balance. So I take it back in and they rebalanced the rear tires. Same thing. So next time they take it to a shop that has a road force balancer and do all four tires and that guy thinks that the 285's might be to wide for a stock wheel. I drive the truck home and the same problem is there. They then order me a set of 265's (Bridgestone Dueler A/T Revo, both times) They put those on and sure enough same vibration but at a higher speed (high 40's) I have not tried to put a load in the bed of my truck to see if the vibration goes away but don't feel like I should have to have a load to make it ride like it use to . I would like to put the torsion bars back down to original height but since I didn't do the adjusting up I don't know how much they turned the bolts on the adjusting key. Any help on any of this will be much appreciated. Thanks