If you did it in the burnout, then I would think it has to be in the Driveshaft...Maybe a U-Joint Failed maybe the driveshaft was bent?
There wasn't alot of load on it during a burnout. So I am guessing you had the 7/8-1" of play for the driveshaft also. I had a U-Joint Break the Rear one twice..about the finishline and it did the same thing. I would put my money on a U-Joint Failure. I don't think the mounts had anything to do with it. I run stock rubber shells on the frame and a replacement rubber trans mount. No issue's.