Well in order to save you some trouble, may I say that as mentioned, do not purchase cheap replacements for wheel hubs. I am stilll fighting with these people and the last set I replaced was back in March! The bearings are good and make no noise but the speed sensors are crap.
Good advice, that was exactly what my problem was.
My Tech 2 couldn't pull a code from the ABS system which seems to be somewhat a frequent problem according to the shop manual. But I was able to log the wheel speed sensor data and it turns out I had a bad speed sensor on the right front wheel in an aftermarket replacement hub.
I got them off of ebay but they were supposedly the, 'better' brand and OEM equivelent from what I've seen posted here I think. The hubs are fine, but after 15K miles the speed sensor went flaky.
The speed sensor was fine at speeds above 12 mph, but below that they instantaneously drop down to 3mph while the other front and single rear sensor slowly count down to 3mph. So the ABS system thinks during slow speed braking that the front wheel is locking up and kicking in the ABS.
I did save the original speed sensors when I replaced the hubs. It was a lucky thing I did since it was the problem.
Another problem with aftermarket hub is that the speed sensor is, 'almost' identical to the GM part but not exactly. The OEM speed sensor wouldn't seat all the way in the aftermarket hub. I took a micrometer to it and found the aftermarket part to be slightly smaller. I had to carefully ground down the OEM sensor to make it narrow enough to fit and seat properly. Luckily I was able to salvage the o-ring and reinstall it to seal the essembly afterward.
So another lesson learned from aftermarket parts, you ain't gonna find their supporting parts to be 100% compatible with the OEM. I usually do strictly OEM, but this time I took a chance because the supposed, 'quality' OEM part was half the price of going to the dealer for a pair. Had I not been able to modify the OEM part to fit I would have been stuck with a crappy replacement from the aftermarket manufacturer, if I didn't have to fight them for it as one poster here mentioned.
So OP, if you have a decent scan tool see if you can do any data monitoring and view the speed sensors while you brake. You might have the same exact problem I did.