I think GM had issued an AD for this problem on the Silverado. I checked for this on my truck - even removed and reinstalled the speed sensors - but no change. My problem seemed to start shortly after I had to replace the front rotors (didn't use OEM rotors...too pricey). I didn't notice any cracks when I bought them and they looked identical to OEM, but maybe the tone rings weren't built and/or installed to factory specs.
Anyone know the gap specs between the tone ring and sensor? If the rings and sensors are in good shape, but the gap is off, would that cause the ABS to fail only at slow speed?