The National Highway Traffic Safety Administration is investigating a Ford Motor Co. recall of more than a quarter-million Explorer SUVs in the U.S. The probe comes after the administration received complaints about repairs intended to prevent the vehicles from unexpectedly rolling away even while placed in park. The problem, ascribed to fractures of a rear axle mounting bolt that could lead the drive shaft to disconnect, was addressed with a software update designed to apply the electronic parking brake if the drive shaft failed, the agency said. But two Explorer owners complained their vehicles behaved erratically following the repair.