2003 Envoy 4WD with 125,000 miles: In automatic 4WD sometimes I hear a thumping sound/jerking. It is intermittent. When I stop, place vehicle in park and then move again it is ok in automatic 4WD. What is the problem? I have never had the transfer case serviced. Do I need to change the fluid? One web site said it might be a sensor problem or wiring that is corroded.