Most of my slippers squeal when they slip, so there is a good chance it isn't the slipper. Could be the pinion/spur but if they are grinding you should be able to see a flat spot on the spur. And it could be an internal gear or the differential as you noted. If you made a video with it running and making the sound perhaps we can help you trouble shoot it.

Sometimes a rock gets where it shouldnt be and will make a grinding noise as well.