VEHICLE WAS IN MOTION- WHILE DRIVING ALONG I HEARD AND FELT A LOUD NOISE (BOOM) FROM REAR OF VEHICLE-VEHICLE THEN SWERVED WILDLY OUT OF CONTROL SPINNING AROUND AND TAKING DOWN SIGNS, MAILBOX-AFTER LOOKING AT VAN IT IS APPARENT THAT THE REAR AXLE SLID BACKWARDS IN WHEEL WELL. WHILE BODY IS RUST FREE, UNDERCARRIAGE HAS EXCESSIVE RUST IN SPRING HANGER AREA-VAN HAS BEEN PARKED ON PAVEMENT ITS ENTIRE LIFE.