Does this bulletin from GM sound like what you have going on?
#02-05-25-006B: Antilock Brake (ABS) Activation at Low Speeds (Clean Wheel Speed Sensor Mounting Surface) - (Jan 5, 2006)
This bulletin is being revised to update the correction and warranty information. Please discard Corporate Bulletin Number 02-05-25-006A (Section 05 -- Brakes).
Some customers may comment on ABS activation at low speeds, usually below 8 km/h (5 mph). Upon investigation, the technician will find no DTCs set.
The cause of this condition may be an increased air gap between the wheel speed sensor and the hub reluctor ring due to rust and debris built up on the sensor mounting surface.
Measure AC voltage and clean wheel speed sensor mounting surfaces.
(Canadian P/N 89022218)
(Package of 2 cans)
Rust Penetrating Lubricant
(Each can will service 50+ vehicles)
(Canadian P/N 993037)
(Package of 12 tubs)
Wheel Bearing Lubrication - Tub
(Each tub will service 25+ vehicles)
Place unused material on dealer shelf for future use.