what might cause the rear passenger wheel to lock up

check the brakes- the caliper might be freezing up-

morels: jamnblues is wright,the piston inside the calipers,has a rubber piston boot, and when they stop moving back and forth,most of the time, they will freeze fully out, creating a vise effect, on the rotor. Take the caliper off, just two bolts and try to spray some WD-40 IN THEM, THEN TAKE A 5-6" C-CLAMP,WITH THE BRAKE PAD ON IT,AND TRY TO SQUEEZE IT BACK IN SLOWLY,sometimes, it will free it up, But first take your cap off the brake reservoir, to let the excess fluid go back in the tank,without any resections.....hope this helps