They've all got slack in them, just how much? U-joints wear, the transmission parking prawl has slack in it, the spider gears get slack in them, the spider gear cross shaft wears out.. You need to find out for sure where you slack is and how much. You can put a chalk line on the outside diameter of the tire with just one wheel off the ground and see how far you can roll the wheel forward and back. Be sure it is all rear end slack and not other areas.