Help me diagnose a vibration
 

Over the weekend a vibration in the truck started. It is a constant rotating vibration, which sounds almost exactly like the previously bad u-joint I’ve had in the rear driveshaft but it doesn’t seem to be the driveshaft. It matches the rotation of the rear wheels, turn for turn. When slow it's slow, fast it's fast.

I put the rear wheels in the air and spun the driveshaft by hand and it sounds like it is coming from the rear of the tranny or the transfer case. The transmission is still shifting normally; I haven’t tried shifting into 4wd yet. You can actually hear the grinding movement from that area.

Any clues? I’m stumped.

does the driveshaft wobble when it spins? if the driveshaft is spinning straight, i doubt it would be in the driveshaft/tranny/transfercase. Perhaps your tires are just out of round, misaligned, or misbalanced?

Tires were balanced less than 1000 miles ago, I can hear/feel the vibration when I'm spining the driveshaft by hand with just the rear tires in the air. That at least knocks out all the front end.