I have a 2000 Silverado 4x4 single cab with a 5.3L engine and 82,251 miles on it. I now have the dreaded "service 4X4 message". Truck was in 2HI at the time. I tried switching to 4HI and then back to 2HI , but it now switches back to 4HI automatically. 2HI blinks but won't stay there and returns to 4HI. 4LO just blinks with 4HI remaining solid light at same time.I can hear transfer case changing under truck(kind of a slight grinding noise, going on then off). when switching to 2HI only.Auto 4WD light will remain on when engaged, with all other lights going off as usual. I bought the truck in '03 with 43,000 miles on it. As long as I have owned it, it's never been out of 2HI. Truck was parked when I did these tests. I would appreciate any help or ideas and your time.