Where is the anti lock brake sensor located on a 1999 Chevy Tahoe?

The ABS sensors on a 1999 Chevy Tahoe are located at each wheel, within the wheel hub assembly. There's one sensor per wheel – front left, front right, rear left, and rear right. They're typically embedded in the wheel speed sensor tone ring (which is part of the hub assembly). You won't see them easily without removing the wheel.