Brake Loud Noise problems of the 2011 Ford Flex

On my way home from work 7-14-2014, entered the exit ramp to gratiot north bound and had to stop for a light. When the light changed green I eased off of my rear brakes and heard a loud noise coming from them and could tell that the brakes were seizing up a little. So the next day and there after I had no problem with them. On 7-20-2014 the did the same thing but not as bad. Made appt. With firestone auto care on 7-25-2014 to have a brake inspection done all four tires, especially the rear passenger brake. I was told that both rear caliper pins had ceased up on the rear brakes not allowing the caliper to open and close properly. This caused the brake shoes to be replaced, both rotors and calipers, because by know it was metal to metal. There was no work done to the front brakes because they were still in good shape I was told. I had this vibrating or easy shake that would happen to the front when I would slow down for a stop. I had this checked back in the winter to see if there is a problem with my brakes but could not find nothing. Not that the back brakes are fixed the vibration is gone.