Lead shot. A bag used for reloading shotgun shells weighs in at 25 lbs and occupies a space roughly 6" x 12" x 2"
I would never use any heavy solid object for winter weight. Not after seeing what a 30 lb battery will do to the tailgate and header just by hitting the gas or brake too hard and having it slide.
Sand bags are good as long as you keep them from freezing, but they do take up a good bit of room. Putting the weight in front of the rear axle behind the cab, you need more weight to do the job. Putting it behind the axle you need less weight, but with a bedcover, it's in your way.
My personal preference with my Ranger is to run 100 - 150 lbs of lead shot (one bag behind each rear wheel and the rest in front of them on the sides) and around 50 lbs of tools (my usual amount). I run a good aggressive AT pattern tire and it's worked well for me. If I get into deeper snow and loose some traction, I shift weight towards the cab in the bed, and if I get on hardpack snow or ice and need more grip, I'll shift weight to the tailgate area. But 98% of the time, it works without moving weight.
And don't worry about the lead shot in a wreck, those bags absorb so much energy it isn't funny. I know because I was in a wreck (not my fault). When I got stopped my tools were scattered all over the bed. The lead shot had hardly shifted positions.