2003 Chevy Astro ABS problem on 2003 Astro AWD

I have a 2003 Chevy Astro AWD van with 69000 miles. The ABS light started coming on at odd times and then going off. Also at times, the ABS would kick in when braking at a normal stop. It makes a "BZZZZ" sound and is sometimes hard to stop. The light coming on and the braking problem are not at the same time usually.

I took it to a trusted mechanic and they said the light code was giving several different errors, so they tried cleaning the contacts to see if that would fix it. They said if the wheel bearing needs replaced that the part alone would be over $400.

It was all right for one day and then started with the same problems, though less frequently. Someone else told me the sensor for the ABS is in the wheel bearing and that is why it is so expensive.

Is this true about the sensor location and price? Is this something we could fix ourselves? My husband is quite experienced at older model repairs.