Pinion bearing issue??

Over the last few weeks I've started to hear a hum/whine that pretty clearly is coming from the rear of the truck under certain conditions. They are: driving on flat or down hill roads, speed between 45-60 mph. If I take my foot off of the gas pedal, I hear the hum/whine. If I press on the gas pedal even slightly or apply the brakes even slightly the noise goes away. I've done some research and it seems to point to the pinion bearing preload.
Anyone had this issue before? Any other things I should be looking at? I plan on double checking the fluid level this weekend.

 

I’d check the u joints first, that is a common noise to hear when they go bad, let off power and hear hum

 

I'll add those to the list.

 

In my experience its always been the diff carrier bearings going bad with that type of noise. If you go from coast to in the gas is there a bit of slack/clunk when transitioning? Thats also been a symptom with a carrier or pinion bearing going out. Could drive it around the block and open the diff filler and dip something into the oil inside and see if you see metallic sheen, obviously if a bearing is going out theres going to be some suspended metal and there will be some metallic shine to the oil as well

 

I really hadn't noticed any vibration, shakes, clunks until today. On the way home coming through town I tapped the gas a few times while coasting and noticed a small "clunk" from the rear end. Not sure if it was the driveshaft or axle. Also not sure if it was always there and I just didn't notice it until after I had read your response. Also not sure that I think it was bad enough to be considered abnormal for a sick up truck. With all that said, I'll add checking the fluid for shiny stuff to my list.

 

Checked the u joints and all is good.
Decided to buy a new drain pan and drain the oil for inspection. Not so good. The magnet had more on it than I would have expected after 30000 miles, much less the 7500 to 8000 since the last time I changed it. After letting the fluid settle for a bit I poured it into another container. Probably 50-75 specs in the bottom of the pan and that's in addition to several that flowed out of it. Guess its time to look for a rebuilt 3rd member somewhere