2000 Cavalier - ABS, Trac Off, and Service Wrench

Hello all!

I have a 2000 Cavalier LS, automatic with ABS, Trac and all the bells and whistles with 155,000 on the clock.

I have been battling a problem with the ABS system for going on 2 years now and I'm getting a little bit desperate.

Symptoms (other than the idiot lights): disproportionate brake pressure to the two back wheels, causing the rear wheels to lock up well before the front wheels.

I have an Actron scanner, and it reads 6 codes:

C1223 - LR wheel spd sensor input signal = 0
C1227 - LR excessive wheel spd variation
C1233 - RF wheel spd sensor ckt open or shorted
C1234 - LR wheel spd sensor ckt open or shorted
C1254 - Abnormal shutdown detected
C1275 - PCM requested ETS to be disabled

I have removed and inspected the wheel speed sensors, and did not find any obvious breaks in the harness wires. All speed sensor harnesses read 4.9 volts across the two pins with the key on. I have removed the rear harness, stripped it down to the bare wires, and tested for continuity while wiggling the wires along the entire length. I'm out of ideas.

What would be your first suspicion/course of action?