Adding weight to the bed (or rear cargo area or trunk) works for improving rear wheel traction in a rear wheel drive vehicle. We used to do this in our 1988 Mustang and in the 1971 Vista Cruiser I drove in high school. It is especially helpful for snow. I suggest that you use smaller 10 pound bags of sand instead, bags made of weaker\fragile plastic garbage bags that will disintegrate in an accident. Put them directly over the rear axle for maximum effect but not behind the rear axle, which lifts the front end. You really don't want to get hit by either concrete blocks or sand in an accident, but the bags of sand will tear apart and the sand will be released a bit slower, and not hurt as much. Its dirtier, but safer.