It happened on my 8 3/4 open rear years ago...the way you describe the noise sounds similar but my noise was very loud resonated through the driveshaft to the 4 speed shifter, I thought it was in the trans at first but I turned my head and the angle of my ear indicated it was coming from behind. Id say if you don't find anything in the drums check the diff. If you don't want to go in right away you can put the rear on stands turn the rear tires in neutral see if you hear anything that way...or if you want start it up and put it in gear let the wheels spin while up off the ground etc.
