This is a common problem on GM vehicles. I don't know if GMT800/GMT900 are setup as others but it woiuld seem logical that they would. There is a wiring harness that goes from the hub for the speed sensor to send signal on the state of the wheel. Voltage is reported back, negative or positive, based on speed. If the voltage is detected to be too low or too fast, the vehicle may engage traction control, Stabilitrak, ABS or a combination, depending on how your vehicle is equipped and what the computers detect.
If the voltage changes due to a short circuit or failing connection, you move out of normal voltage, either dropping or spiking. This sudden drop or spike is "translated" by the computer as a sudden application of a singular brake resulting in what it thinks is a lock up, or sudden wheel spin that indicates loss of traction, which is translated as a skid.
In reality only the voltage change, you're just driving down the road. So for no reason at all what you experience is the ABS engaging on a gentle braking for no reason, or the traction control or Stabilitrak engaging while you're just driving down the road. Most commonly happens when you're starting off.
The fix is pretty simple - but again - I've only dealt with this on non-GMT800/GMT900 vehicles, but the symptom you describe is identical. You need to see if the harness is plugged in. It can be as simple as finding its location, unclipping, cleaning, and reinstall. If the wires are corroded or bad, then the harness needs to be replaced. on W-Body and U-Body vehicles they are cheap - about $25 a corner and if you are even remotely handy with solder, shrink tube, sealing, you can do it yourself. Much easier on an Avalanche as you can wiggle under the truck without a lift.
What I don't know is if those sensor wires are as exposed on GMT800/GMT900 as they are on other GM vehicles (W-Body, Theta, U-Body, Epsilon I).
The problem is you may very well have a code that says your right front wheel speed sensor is failed. They replace the hub, which is a lot more than a $25 wire with a plug on it, and you still have the problem. Then they see your left front is bad. You replace that, and still have the problem. The hubs etc. likely aren't bad - the problem is the wires running from them have gone bad.
Good luck - and I'm hoping someone will jump in and either go, "yup, some GM vehicles have this problem but the GMT800/GMT900 vehicles got different treatment," or, "here is a schematic to show what Chief is talking about. Start here."
