VEHICLE WAS IN FOUR WHEN DRIVE LOW STOPPED AND GOT OUT TO OPEN A GATE HEARD A LOU POP LOOKED BACK IT WAS ROLLING BACKWARDS WENT ABOUT 200 FEET WITH A TRAILER HOOKED TO IT THE TRAILER FLIPPED AROUND HIT THE SIDE OF THE TRUCK MADE ANOTHER LOUD SOUND AND IT STOPPED. WE ARE BEING TOLD NOTHING IS WRONG WITH IT. ANOTHER TIME HAD IT IN LOW FOUR WHEN DRIVE AND IT KEPT ROLLING IN PARK. THE VEHICLE WAS PARKED. AND WHEN WE GOT TO IT THE GEAR SHIFT WAS STILL IN PARK.