Here's the deal, I don't build things that the aftermarket is full of. I modify mounts, I fix the problems with existing equipment, I fab parts that don't exist.
If the tire rack bumper is stock, fix the body mount on the rear quarter: open the quarter panel from the inside. Bolts, nuts, lock washers, and 2 pieces of 3/16 plate. Sandwich the sheet metal ... I think you get the point, but the internal plate can designed to cover more area than the external plate, thus providing more support. If it squeaks, lube the joints and wrap electrical tape around the tube that locks in the bracket.