Thats what they "thought" too I'm gonna go knock on some wood now since my past 2 truck have never had that problem...

 

im with everyone else on the death wobble what it sounds like to me..... also try adding some 6647 bilstein shox in back nice addition anyway to ride and you loose that harsh rebound snap after speedbumps etc...

 

I been there and this is whats wrong. The rubber bushing is worn out on the bar that locates your axle. for some reason i cant remember what its called right now but i think drag link. Anyway, it runs from the frame on one side of the truck down to the axle on the other side. Locates the axle side to side. They want $180 for it at the dealer but you can buy just the rubber at NAPA for $13. Your steering is probably tight, its your whole axle moving left to right thats causing the shaking