It is possible that the Front axle shift motor actuator or associated vacuum hoses and / or switch may have a leak. Carefully inspect all vacuum hoses for splits, cracks and swelling. Replace if necessary. There are (5) components, !. Vacuum Control Switch, 2. Air Vent Filter, 3> Vacuum Shift Motor, 4. 4WD Indicator Light Vacuum Switch, 5. Vacuum Check Valve. plus vacuum harness and hoses.