Many years ago we used to take a couple of tractor wheel weights, set in an old car tire to prevent sliding around, right at the back of the truck box. Never gave a thought to what might happen in a sudden stop but we got lucky. Also had a couple of those steel wheels off the old one way diskers complete with the solid cast weights. They helped but again, unsecured weight just sitting in the box that could have been dangerous on a sudden stop. Now I don't drive the 2 wheel drive pickup much in winter. Usually has the 100 gallon fuel tank in the back with at least a little fuel. That helps, plus carry a shovel if I happen to get stuck.