If you have an electronically activated 4wd, it is the switch in the dash. The one with the Auto 4wd, 2WD, 4wd hi and 4wd lo. Especially if you see the lights on the button not working occasionally. I also have a 2000 GMC sierra and replacing the switch and it fixed the problem. I also know several others that had the same problem. The ABS code may be the wheel speed sensor. GM put out a recall for this last October. I had my truck fixed for the recall. However their fix is to clean the old one and spray it with rust prohibitor. My wife was driving the truck later that week and the problem reoccurred after they fixed the recall problem. The anti-lock brakes can engage on dry pavement between 3 and 10 mph which can cause what GM described as a rear end collision. Which is what happend to my wife.
Good Luck.
Tuesday, March 7th, 2006 AT 4:31 PM