Welcome to Allpar. Rear disc or drum?
Are the parking brake shoes or cables stuck or are the regular brake pads/shoes stuck?
If it is the parking brake sticking, try wiggling the rear cable into the drums if they are drum brakes. Or tap the actuator arm inward on the backing plate if they are rear disc.
If they are rusted, they will have to come apart to be freed and lubed. New hardware, etc. may also be needed. You want fully working and safe brakes.