I ground mine down with a 4" grinder. Pretty slow, about 45 minutes per drum, a 6" er would have been nice. Put one rear at a time on stand, chock truck real good, remove wheel and run about four of the lug nuts back on to hold drum on in perfect alignment. You are going to put truck in low gear, engine running and grind on the material while drum is turning. You need to have the grinder braced on something firm so that it will grind evenly and feed it into the drum against the direction it is turning. If you were to feed it in the direction the drum is turning I imagine it would take your grinder and spit it out the other side and perhaps start all other kinds of havoc. Be careful. I used the removed tire to guide the grinder while I fed it toward the drum.
Be sure to put the lug nuts on as I already mentioned. The drum may be on pretty tight you think, but if it comes lose and gets out of alignment while the truck is in gear it may just lock up against the brake shoes and cause your truck to drive right off your stands. I had that happen one day when I put mine in gear sitting in my truck. Just fooling around, don't recall exactly what my intent was at the time but the truck all of a sudden lurched and I just barely got the brakes before it drove off the stand. The drum was tilted sideways and jammed pretty tight.
Back to grinding the drums. I have about 1/8" clearance around my drums now and could remove even more material. I wish I knew how much space is adequate for cooling and how the hot drum is affecting my tires. It seems to be fine as is. I got the brakes pretty hot one day and felt the rim on the inside and it was only warm but the real test would be going down a long grade or doing it repeatedly in some kind of situation. Dunno.
The improvement in my truck is wonderful. I drove it for ten years with the factory rims not knowing just how out of round they were. The truck now runs smooth and handles better. One extra I did that made part of the difference. I had the tires balanced on a Hunter Road Force balancer. Some say this is not a cure all but it did remove a small amount of jiggle that I was still feeling after putting on the new michelins and having them balanced on the standandard balancers. My truck rides silky smooth like I have never seen before.
Be sure to keep the grinding dust away from painted surfaces, it will attack the paint and make you very unhappy.
If you have a machine shop and money go there.