Denny, I vote for the "out of roundness" of the drum or the brake shoe material. Its been a while since I worked on the old stuff....except for a recent Ferd, but brake shoe composition is a big factor these days. I recently changed some on a car three times before I got rid of the grinding sounds.
I have not got to drive my panel yet so I do not know what mine are going to do..... I have a set of extra drums that are turned but unfortunately they are for a 1/2 ton.
As an after thought, if there is a big tractor trailer shop in your area they are surely getting brake shoes recored by someone fairly close. If you could find out who those people are you may get what you want. I know there is one in Charlotte, N.C. ...or it was there not long ago. They can reline your cores and true then anyway you want them....with good stuff.
