VEHICLE WAS PARKED ON ICY DRIVEWAY (5 DEGREE ANGLE). IN PARK. VEHICLE WAS FOUND AT THE BOTTOM OF DRIVEWAY AGAINST A TREE. THE WHEELS WERE TURNING AND NOT SKIDDING AS PROVEN BY TREADS IN THE SNOW. I BELIEVE ONE OF THE REAR TIRES MAY HAVE SLIPPED AND SOMEHOW THE OPPOSITE DISENGAGED AND ALLOWED IT TO CONTINUE BACKWARDS. THIS HAPPENED WITH MY 2002 GMC SIERRA. THE EXPLANATION I WAS GIVEN IS THE DIFFERENTIAL SENSES A SLIP AND THE OTHER SIDE ENGAGES ALLOWING IT TO TURN BUT IF THERE IS A SLOPE THE VEHICLE WEIGHT ALLOWS IT TO KEEP ROLLING DOWN HILL. IF SOMEONE IS BEHIND THE VEHICLE THEY COULD BE INJURED OR PINNED BETWEEN ANOTHER OBJECT. YOU CAN PARK ONE ON AN ICY DRIVE AND NUDGE IT AND IT WILL BEGIN ROLLING BACKWARDS. THERE SHOULD BE A PRONOUNCED WARNING OF THIS OR AN ENGINEERING SOLUTION