OK, After checking again, I did see a rub mark on the outdrive faces on both sides. Oddly enough, I couldn't see any marks on the Spur.
I filed down the plastic ledge for the diff rings until the "scratch" was gone, then I went a bit further to be sure they didn't rub again.
Tried the tc4 2day at the track. The Plastic rear diff Still got warm and tightened up. So I gave up, bought the steel OD's, and rebuilt using the old thrust bearings, washers, and spring. I had to use new diff rings, but used the old balls and spur. So.... The ONLY thing that was NEW in this rebuild is the rings and steel outdrives.
It now works fine. Nice and smooth like Butta', loose without slip, and never got hot after 3 heats and a Main. I still don't know what went wrong with the plastic diff, but I'll agree that the metal ones just don't do it, for whatever reason. Hmpphhh.....
BTW, The new steel diffs needed to be tweaked to work with the stock bearings cuz the spindle was too big. After chucking it up and sanding it down, we got the bearings to work, but it was a major hassle.