Noisy Rear Gears, or What

If it's a constant rumble sound, it's probably an axle bearing. If it has a whine and/or changes pitch when you accelerate/decelereate, it probably ring and pinion. If it's pinion bearings, you can usually feel them bind as you rotate the pinion flange by hand. Pull the driveshaft loose and rear drum off first so nothing else can be binding. You can usually narrrow down the location by getting the car up in the air and checking different locations with a stethescope or wooden dowel up to your ear while the car is running in high gear. Just make sure the car is securely on stands with front wheels chocked and nothing/nobody in front of the car that you don't want to run over! Oh, bad gears and/or bearings don't repair themselves....if they're bad, they won't get better, only worse.