My two cents: I think it all goes back to the rebuilder. I would first change the balancer, just in case it did slip. If not, then I would drop the oil pan and windage tray and check the crank, either forged or cast, then, with a flash light if possible check the connecting rods numbers if matching. Are all the bolts on the balancer the same size or missing one or two? Is a incorrect balancer pulley being used and not centered. Not sure of the reason for the "DOT" on the stock flexplate, but it should be opposite of the plug on the converter, is my understanding.
