I had a similar problem with my 07 GMC 2500 HD .
Is your 5 th riding level when hooked-up ?
mine wasnt , and I had to have my trailer axle's turned .
My trailer was making contact with the top rail of my p/u box when backing up into a spot where the ground was not completely level . Turning the axles on my trailer has solved this problem , and I would highly recommend doing this.
I should add that raising or lowering my hitch would have made my trailer unlevel.