Could be the rear-end , might want to check the gear oil level in it, once you remove the plug oil should run out of it . if not stick your finger into the hole you should be able to feel the oil , if not you will need to add so to it.Add some 80 - 90 w gear oil, you can buy this at the auto parts store in a plastic bottle maked so you can put it right into the hole of the rear-end.
