I added the support in the area where the rubbing took place. No pix right now because my AS is about twenty miles away for service. No parts number either, but it was obviously cut from an extruded aluminum channel piece. It is about 3 to 4 inches long, and in cross section is an inch deep u-shaped channel with a 1/2 inch flat extension at the top and on each side of the 'u'. The flat sections on the 'u' fit against the surface of the trailer, and are drilled and riveted there. The bottom of the 'u' fits against the guard, and is drilled for a short bolt, with a stainless steel 'finish nut' to the outside surface of the guard. [I know a picture would do wonders.]
[The AS people seemed to be really aware of the issue at the time, so I assumed they would make design changes to take care of it in the future!]
I honestly believe the rubbing occurs because the guard flexes when you are at highway speeds, enough to force it against the trailer body. I could see movement in the surface by watching in the rearview mirror, but don't have any now. . . and no sign of any further working of the guard at the points where it is fastened. Sure worked for me.
1996 28ft Classic
2008 Silverado Duramax 2500HD