99' prerunner whining from backend

I have a 99 prerunner that whines from the back end when i let off of the accelerator going between 45 and 75 and will stop when i hit on the brakes. I changed my diff fluid and checked the drum brakes and the pads were not worn and everything appeared normal. Anybody have any suggestion to what it might be. This would be greatly appreciated.

I suspect that the gears are set up to tightly. Not a huge problem but may affect longevity. I would guess that you recently added a camper shell and that space amplifies the gear whine (like a speaker box.)
Best I can do with the info you gave.