Just about every state in the Union has to worry about vehicle rust in one form or another. But obviously a 4x4 body and frame will last decades longer in Arizona than it would in Michigan.
Besides parking and not using your 4x4 in the winter (which really isn't an option for most of us) what do you do to make the steel body and frame of your 4x4 last longer in areas with harsh winter roads polluted with salt?