You don't need a lot of weight, you'll affect your milage, course spinning your wheels makes for zero mpg. A few bags, 2 to 4, of sand from a Home Depot type place should do it. Put um right over the axle across the bed. If you do get stuck you can spread some of the sand under the tires to gain some traction. Mebbee put the sand bags inside some plastic bags to keep it dry so it don't freeze into a solid block. You could also put a few cinder blocks in the bed too,just enough to go across the bed, one layer, also right over the axle, put the sand bags on top of them, cover with plastic. Turn um across or forward, depending on how many ya want or can afford. Next spring you can use the blocks and sand to make a litter box for an industrial strength cat. If you already are buying cat litter you can use that instead of sand; just get the bigger heavier bags; and the cheaper, more clay like, stuff works better than the stuff with fancy additives. Of course sliding down the highway with the wheels locked and not turning at all improves gas milage considerably, if your turning 360 do-nuts at the same time it makes your commute much more interesting. The stops can be rather abrubt tho.

200 lbs of sandbags or 1 irate mother in law!

My son uses about 4 to 6 big bags of kitty litter and if he gets stuck he opens the bags and puts it under the tires for grip.