I could have used a locker last weekend. Amazing how a 8000lb. truck can slide so easily backwards down a mild gravel road hill, then sideways, spinning the whole way. Of course this was a road which was covered in ice at the top of the hill, but only on the right side. Which tires spun? The right side, just enough to let the truck loose traction and slide sideways. Scared the bejeezus out of me looking in the right rear view and seeing the trailer I was towing next to me. I will be one of the first to purchase a locker for the rear when available.