Rear passenger wheel moving slower than drivers

[Ctownford22]

How's it going guys. I have an 03 stock Explorer with 150k miles. The trouble I'm having is this. My front 4x4 auto is staying on continually. I lifted the vehicle on 4 stands removed rear tires calipers and rotors. When i put the truck in drive at idle, I noticed the passenger side hub was moving a lot slower if at all, than the driver side , but looked normal when the gas pettle was pressed. My question is, could this be the wheel bearing, the rear diff bearings or more seriously the diff gears? Thanks in advance.