When rebuilding my 53 1/2 ton, I discovered several nickle-sized holes in the frame rail just beind the driver's side of the cab. It looked more like acid had eaten through than rust, and in fact the rest of the frame had no bad rust. I cut and fit 1/4 steel plate...one piece to fit across the top and another to fit along the outside of the rail and welded them on. Had to be careful to allow clearance for the parking brake mechanism and bed bolts. If by "rear", you mean the tail ends of the frame or the arched brace in the rear, it would seem that anything that would take a smiliar welded patch would work, since there is less weight in the rear and torque less of a consideration than up front.
