I was plowing snow today, and noticed a lack of power from my rear wheels. They turn around, but under any load, they don't seem to provide much power until I engage the differential lock. Then they work just fine. It drives fine on dry pavement. If they did not engage at all, I would assume something is broken, stripped or worn. The fact that they work fine, and provide plenty of power when the diff. lock is engaged must be a clue? Thanks!