After more Google time, I see this is a common problem with multiple possibles.
Can't find any wires from the back wheels like on the fronts as in the video showing the fronts. Unclear to me if the back ones even have sensors???
This guy with a good report found his problem was in a ground connection or else some other wire connection. He did a bunch of things and can't say which thing he did worked. I will check my grounds (was mentioned in a post above too)
https://www.gmtruckclub.com/forum/threads/2003-suburban-abs-light-issues.96992/