+1 on warpanda. The rust should break free with all the weight on the wheels. I'd try to rock the car. First forwards, then backwards.
Heres my best explanation as to why:
Or for a more simplified example, hold your hands in front of you, fingers towards the cieling, thumbs towards you as if you're awkwardly holding a plate. Your thumbs are basically the pivots. Now imagine that there's a ring just outside of your hands...that's the drum. As the brake is engaged your fingers go out, as if the plate grew, and press against the inside of the drum. Driving one way will tend to tighten one side by dragging it into the pivot, but pull the other side away from the pivot. So you need to go forward and reverse to brake both free.
Other options:
1) Tap vigorously with a rubber mallot to brake things free. Esp when combined with
#2
2) There are two holes (IIRC) that you can screw a bolt into in order to remove/replace the rotor/drum. Trying to get things broken free laterally like that might work.