I just finished changing my rear rotors and brakes. They also squealed when I finished! So I took the calipers apart, replaced one guiding pin that froze up and greased both pins with anti seize lube. Now the brakes squeeze evenly to the rotor. I also re-glued the loose shims to the pads. I don't know which one of the problems was responsible for the noise but so far the squealing has gone away.
