I have 00 Silverado with the Auto-Trac T/C. The service 4wd light came on however when it came on it only resulted in the inabilty to transfer out of whatever mode i was in (2hi/4hi/4lo/auto) and as soon as i turned off the truck it would go off. One day i was driving the truck on auto mode and it started grinding but it stopped after i switched to 2hi. Now everytime that i drive in auto mode at speeds above 55mph it grinds only when i press or depress the accelerator pedal. Any one have any clue??