I've got a 99 XLT 4WD auto V6 3.0L, whenever I take off from a stop and the ground is wet (rain or puddle), my rear tires spin and I can't go for a few seconds. I also have a problem when I'm turning and hit a bump, the rear end wants to fishtail and I feel like I don't have much control. I've heard that it's a matter of the bed being to light, if that's the case how would I add permanent weight to the bed?