Rear End Humming Noise

At highway speeds, usually around 55+MPH, there is a humming noise that comes from the rear end of the truck. It only makes this noise when I give it gas. Once I let off, the noise stops. The dealer replaced the rear end and it stopped making the humming sound but, 600 mile later and it starts again. The dealer said the would post a report on the problem. Anyone have any ideas?

 

This happen to me in my 07 sport after I got my 4.88's put in. I took it back to the shop I got the work done at and they fixed it right up. Anyways the problem was a burned out bearing in the diff.