The rear upright is a 2x4. I went with a 2x4 instead of a 2x2 here to help support the two middle boards, the crosspieces for which float free. I've cut the bottom of the support to sit on the wheel well, and put a notch in to accomodate the crosspiece for the lowest board. They are secured on the other side with L brackets. As with the forward rack, the uprights transfer the weight they carry down into the van, not into the brackets. I will replace this photo with one in better focus soon.