Constant replacement of rear rotors

First i would the car to a professional auto repair shop and have the brakes evaluated. you may have to pay a small fee, but it will pay for itself in the long run. it sounds like the rear calipers are sticking or dragging, or the parking brake is also dragging. the rear calipers are actuated, they stop the car in normal braking and they also are used when the parking brake is applied, the caliper piston will move out and keep the car from rolling when the parking brake is applied. worst case senerio you may need to replace the rear calipers, caliper brackets, rear brake hoses and both rear rotors, unless they can be resurfaced. the vibration or pulsation is more than likely excessive runout or thickness variation in these rotors from the calipers dragging (not releasing). also if the car takes too much effort to stop and you have had to replace these rotors 4 times, that is not normal. have a repair shop check to see if the brake fluid is contaminated. a good and quick sign to check for contamination is if the rubber under the brake fluid cap is distorted.please do not drive this vehicle until it is checked out by a professional auto repair shop. thanks