Get the rear end up in the air, both rear tires off the ground and put the tranny in netural. With jackstands supporting the car, get under it near the rear end and rotate either rear tire. Listen and see if you hear the grinding noise and can determine where it is coming from. If it lound enought that you can hear it with the engine running then you should be able to spin the tire fast enought to hear it. If the noise is coming from the rear end, remove the rear end cover and look at the gears and for pieces in the bottom of the differental.
If the noise sounds like it is coming from the brake area, then you may have bearing problem. Still have to remove differental cover to remove clip so that you can remove the axle, so that you can replace the bearing and bearing seal.
Gary