Wheel speed sensor / abs brakes

2000 suburban, 80k, feels like the ABS brakes kick in at low speeds when coming to a stop.
My local mechanic took a look at it, he drove it and felt it also. He hooked up is code reader and said no codes were displayed. Also, no ABS dashboard lamps lit either. He said the speed sensor is probably bad, and that its part of the hub assembly, which is expensive.

Looking at my Chiltons manual, it says the speed sensor bolts to the hub and can be replaced by itself, but there is also a toothed sensor ring in integrated with the bearing, and if the toothed sensor ring is damaged, the entire hub must be replaced.

Is this a common problem, and is the problem more likely to be the sensor itself, and like the manual says, can it be replaced, or like my mechanic said, must the entire hub be replaced (if it is just the sensor)?

Thanks