If it were the rears seized on the car would not move freely after being left with the handbrake on!!
When I bought my car there was 1 freely moving piston out of four in each front, the rears were ok.
All I can suggest is to take the pads out, putr a thin piece of wood between disc and piston and work in/out a few times, sometimes works sometimes not.
BIGGRED.CO.UK is the man for brakes if you need any.
tabetha