Provides Expedition with an outstanding four-wheel-drive system. The system is vacuum operated. In two-wheel-drive mode, a vacuum is applied to withdraw the clutch ring from the wheel hub coupler, completely separating the axle shaft from the wheel end. This avoids “back driving” the axle shafts, resulting in less wear and drag and slightly improved fuel economy. In four-wheel-drive mode the vacuum is shut off, the clutch ring engages the wheel hub coupler, and the axle shaft and hub are locked together to provide the necessary additional traction.