On certain trucks, the rear axle hub bolt(s) could break and cause the axle shaft(s) splines to wear. If this happens, there could be a loss of power to the wheels. Additionally, if the truck is parked and the parking brake isn't applied, the truck could roll away.
A loss of power to the wheels could increase the risk of a crash. A parked vehicle that rolls away could create the risk of an injury or a crash.
Ford will notify owners by mail and advise you to take your truck to a dealership to replace the rear axle shafts.