Typically, the best way to diagnose the ABS braking system is getting it scanned with a scanner capable of detecting GM CHASSIS/ABS DTC error codes. Like mentioned above it could be any hub/bearing/wheel sensor or problem with the ABS motor/modulator, EBCTM control module or associated wiring.
Most likely a wheel sensor. Check that all 4 are plugged in properly. It is an easy DIY to check the wheel sensors. Using an ohm meter check the resistance of each wheel sensor. They should measure around 1k ohms, give or take.