My 2004 Silverado is having trouble with its 4x4 drive. What is going on is-- when I push the electric button to put it in 4x4 the light changes telling me that it went into gear. However the trucks acts as that didn't actually happen. From a sit still the front tires don't pull. When I get the truck moving it makes the same low noise it always has even when it was working right. Also periodically in get a message on my dash to service 4WD. When this happens the buttons that control the changing from 2WD to 4WD don't word. If if turn on my truck and restart it the button light back up and changes to and from 2WD - 4WD no issues. What is going on and what do I need to do to fix it?