Going down the road I got a severe shimmy and shake from the rear of the truck. When I stopped, there was a burning rubber smell and smoke coming from the rear differential. No clunking or heavy metallic noises, and the axles never froze up. I can put the truck in gear and move it forward and back. We towed the truck approx 10 miles in neutral at 10 miiles /hr. Any faster than that and the shimmy started again. What are the possibilie problems and how severe could it be?