DRIVING ON I90 WEST WITH NO PROBLEMS NOR WARNING THE REAR END SUDDENLY LOCKED UP COMPLETELY, SEVERING THE DRIVESHAFT AND ROTATING THE DIFFERENTIAL SO THAT IT EMBEDED ITSELF IN THE SPARE TIRE (MOUNTED UNDER THE VEHICLE). RESULT WAS AN IMMEDIATE LOSS OF CONTROL, VEHICLE SWERVED TO THE LEFT AND RAN OFF THE FREEWAY INTO THE MEDIAN. REAR WHEELS WERE COMPLETELY LOCKED UP AND UN-MOVABLE. INVESTIGATED BY WSP (REPORT # 0570977). CLEARLY AN EXTREMELY UNSAFE VEHICLE DEFECT. FORD DEALERSHIP (EVERGREEN IN ISSAQUAH, WA) MECHANICS ALL CONCURRED THEY HAD NEVER SEEN ANYTHING LIKE IT.