Muffled rotating sound coming from rear

On Tuesday morning I noticed a muffled rotating sound coming from the rear of my truck. It sounds like and muffler coming down on the RPMs on a manual car. I can confirm that its not the exhaust on the truck because I can put it in neutral and watch the RPMs go down and I could still hear the muffled rotating sound.

Now I have topped off the differential, it was low about .7 Quarts, greased the only two zerk fittings I found on the front propeller shaft, and I checked the rear drums. The only other thing I can do is check the front diff and the transfer case.

I read some other forums on here about a whine but this sounds is not a whine.

Any ideas?

 

Oh, and the truck just hit 10K miles two days before the sounds started.

 

Rotate tires?

 

I'll vote for one of the brake shoes dragging. Maybe pull the hubs and knock out any brake dust build up out.

 

Is it speed specific or constant?

 

It start at 35 MPH

 

I did this last night, there was a lot of dust which I thought was excessive and I readjusted the brakes.

 

I’ll do this tomorrow just to get it off the list.