Rear Tires Suddenly Lock Up Going 65

Transmissions have a function built into them where when you let off the throttle they will use engine compression to slow the car somewhat. Much of this is dependant upon year make model type of trans, etc. Hard to believe if this is what malfunctioned that it is now fine, and didn't tear itself to pieces. Call me stubborn, but I still think this car went into reverse. When you fool around long enough with the shifter in a way that it really wasn't meant to be used, eventually the law of averages will kick in and something bad will happen. I don't know how hard you are getting on this car shifting like that, but maybe the motor mount broke and you didn't notice it because you were too busy trying not to go thru the windshield.