1986 Chevy Cavalier rear brakes sticking

Hi - '86 cavalier hatchback - a great car. Problem is after being parked overnight or while at work several hours, upon start up and drive, the brakes will not always, but about 50% of the time, clunk hard and 'grab' or stick against the rear tires, even causing the tires to squeal - seems more on one side than other, but affects both. Within first minute of slowly driving forward, and trying brakes again, gently, problem seems to have corrected itself and no longer grabs and thunks and braking is smooth and normal.

Is this something that needs immediate attention. Has been going on several weeks.