It could be that the wheel bearing in the front end is bad or that the wheel sensor is bad.
GM had some problems with the wheel speed sensor pulling out of it's bore and not being able to sense the reluctor wheen built inside of the bearing on the front wheel bearings.
One solution was to replace the wheel sensor, which I had quite a few used one in boxes when I worked on those models and the other was to replace the wheel bearing.
The easiest way to tell if the bearing was bad was to jack up the truck with a floor jack under the lower ball joint and wiggle the wheel. If it wiggles - it is bad.
The next step would be to take out the wheel sensor and look at the bottom of it.
If it looks ground off - and there is wires hanging out of it, then the wheel bearing is bad.
If everything inside looks ok, then you look at the flange - if the flange is warped from rust build up under the sensor - then the sensor is bad.
Some vehicles had a sensor built right into the transfer case for the rear wheels and if it went bad, you had to take it to a transmission shop
