



NO CINDERBLOCKS!!!
You want to add weight over the rear axle, but you don't want to use ANYTHING that could become a dangerous projectile in the event of a wreck. I've seen what damage a couple cinderblocks can do to a pickup bed just by sliding around and it ain't pretty. IMHO, I wouldn't want to chance a wreck and have them cinderblocks join me in the cab. No thanks.
Sandbags, snow (if you don't use your bed much or at all, just pack in a load of the fluffy stuff), bags of salt or kitty litter, bags of lead shot, etc. Something that woln't move a whole lot or be damaging (the bags of lead shot surprisingly don't move much and take up minimal space, thus my preference). You'll want to protect whatever you use from getting wet (except snow of course) and freezing into a solid lump, however, because then it could be just as bad as the cinderblocks.
I've never run official snow tires on my Ranger, nor studded tires (around here the law states that you have to buy 4 snow tires or studded tires and run them on all four corners, you're not allowed to have just studded rear tires or just snow tires on the rear.
I ran mud tires on the rear and aggressive AT tires on the front a couple winters. They did great in deep snow as long as I had minimal weight over the rear axle (100-250lbs). They sucked on hardpack and ice, however. With a set of V-bar chains on the muds, it was like having a set of tank tracks on the thing, I had awesome traction on ice, hardpack snow, fresh snow, deep snow, etc. Of course, running chains all the time, especially over mud tires, is a bit of a PITA. I got away with it cuz I only had about a mile to go to campus and I could stay on roads that were always covered in hardpack snow or a thick layer of ice.