This might sound really dumb, but it happened to me after disassembling the differential on one of my Xmods. Check the screws holding your differential casing together. One of them might have pushed plastic from the screw journal (where the screws come through the casing) into one of your gears.

When it happened to me, the car ran and sounded perfectly fine to begin with...I disassembled the diff to lube up the gears, reassembled it without any modifications, and it immediately made a grinding noise when I ran the car. Upon another disassembly, I found that one of the diff casing screws had pushed some of the plastic "flashing" from the manufacturer's molding process into the teeth of the main ring gear on the differential axle. I just trimmed out the excess plastic (and there was barely enough to say so), reassembled the diff, and the car ran fine again. It's just one thing to consider, if you haven't solved the problem yet.