More Rear End Woes

[dennyge]

I posted this at the end of another thread, but thought I had better start a new one as I need some of everyone's expertise on this.

I have a hum coming from what I am fairly confidant is the rear end. I took the truck in and had an additive added to the differential (a Wynns product designed to quiet the rear end) and still have the hum. What confuses me is that the hum is only there when I have the accelerator depressed. When I let up on it and coast, the hum goes away. I assume the differential is still moving while coasting which would mean that the hum continues, but it does not. It is not a tire hum. Any ideas? What parts in the rear end would create a hum only when the accelerator is depressed?

I am really lost on this, so any help would be much appreciated. Thanks, guys!