Sticky rear wheel wears tire

Just took the wheels off my wife's Prius to take to a tire dealer so they can put a new set of tires on them. Found that the rear passenger tire was baddly and very unevenly worn to the cord (in three short spots about 3 inches appart) on the inside half of the tread. We had noticed increasing tire noise in the last few weeks and thought it was a minor flat spot developing on the tires. This tire's tread was worn twice as much overall compared driver's side rear tire. And the rest are wearing fairly evenly.

Her 2005 Prius only has around 52K miles. The shocks are fine and she just had it alligned. So, since I have her car suspended on stands, I tried turning the rear passenger hub and found that it was hanging up significantly for about 30 degrees of its turn. The driver's side did not do this. Parking brake is off.

Anyone have any idea what could hang it up like this? There's no scraping sound like a rock in there, and actually no sound from the drum at all. Almost like the brake fades in and then lets go as the hub is turned. Help, anyone?