You could have had a brake pad detach from the backing. If that is the case, you will need a new rotor (and pads). I just had this happen two weeks ago. New Brake pads for rear=$30 and the new rotor was $22. Took about 15 minutes to fix. Slide under there and check both sides of each rotor for grooving and check the thickness of the brake pads.
2001 TJ Sport Anniversary Edition;1998 5.9 ZJ with lift, armor, 32" MT