Hey guys,
I had the same problem on my 99 Silverado. I pulled out the front wheel sensors, cleaned under them, put them back in and the problem was solved. I think it has something to do with rust moving the sensor too far away from the hub. I hooked up a volt meter, measuring volts AC, while spinning the hub. You should read more than 250 mV, I was getting ~110. After cleaning them I was reading over 400 mV and the problem was gone. Hope this helps!
sparhawk56