Sorta depends on the cause. What have you checked so far? A rear parking brake cable could be rusted in the partially-applied position. If the vehicle has been parked for quite a while, the shoes or pads could be rusted to the drum or rotor.

Try opening the bleeder valve on brake caliper in question, see if brake will release. You can follow brake bleeding procedures. turn key off, disconnect abs actuator and control unit connector or negative battery cable. Connect a transparent vinyl tube and container full of proper brake fluid to air bleeder valve.
Fully depress brake pedal several times. With brake pedal depressed, open air bleeder valve. Maybe that will release the brake pressure. Close valve.

Also,Take the wheel off, make sure there is no ice or anything frozen.

Hey caradiodoc, didn't mean to but in. I didn't see your post. I was looking up info on his abs system. Have a good day, man.

You have a dandy day too.