On the last run, I discovered there rear locker was not engaging.
I could hear the air pressure relief on the front solenoid, but nothing from the rear.
Troubleshooting the issue was weird because my test light was showing voltage on the bad solenoid.
Thinking the solenoid was bad, I tore both down thinking maybe clogged with dirt. No.
So, swapped the solenoids over and the problem moved to the 'good' one.
I put my DVM on the bad solenoid wires, and nothing, just 0 volts. But my 12v test light was still lighting up...very odd.
Eventually I traced the problem to a broken wire under the passenger seat!!
RESULT!! I got my rear differential locker working without having to replace the very expensive ARB solenoid.