Air rifle BB's work best, not only do you get a stiffer sidewall so you don't peel the steer tires off turning when loaded, but also the added weight when empty helps the trk get around much better. The BB's in your rear tires will help lock your tires to the rims so the rim doesn't slip in the tire, get 'em packed in as tight as you can!
I've found that locking the rearmost diff in a 6x4 works best so it doesn't impede on steering too much.