GMC Repair: 07 GMC2500HD bumps & bounces between 40-55, hd truck, cross braces

Question
After taking our 07 GMC 2500HD in for balance and rotating, I noticed it
bumping/bouncing when you reach 40-55mph.  I had wheels checked,put
new tires on and still. The dealership service said nothing is wrong, that it is a
HD truck and will do this.  It didn't do it before, rode like a dream.  Only thing
I can think of, my cousin got it stuck on a fence made of just wooden poles
and barbed wire and had to be pulled out by his brother. He, of course, did
not say anything his son told my boys about this.  Could this may have bent
the frame?  I'm not sure if the dealership checked for a bent frame. They said
they couldn't find anything wrong with it.  Help?

Answer
i would check to make sure there is nothing bent under the vehicle ie; frame, frame cross braces, trans cross brace, control arms.  Also make sure there is nothing stuck in the control arms that would keep the vehicle from getting full extension when jounced, and make sure the shocks and or drive shaft was not damaged