Hopefully you got this figured out by now, but if not;
C81: Steer Unit Sum of Analog Inputs Out-of-Range
Reason: Wiring problem to steering sensor or failed sensor. Open or shorted R2 (carbon potentiometers only).
Tests to Run: 1. Run Learn on the steering.
2. Check wiring between pots and PSU. For trucks with a carbon potentiometer, the resistance of R2 should be 620 ohms.
3.Verify harness crimps are not missing or weak.
4. Verify pot voltages, V1 and V2, following the "pinout Matrix".
5. Verify correct version of software is in the VM, PSU, and TA.
6. Check for shorts to frame.

As for C91, that's missing from my manual as well for some reason. Not even in the contents.

C94: Steer Unit Sum of Analog Inputs Warning Threshold Exceeded
Reason: Wiring problem to steering sensor or steer sensor intermittent.
Tests to Run: 1. Run Learn on the steering.
2. Check wiring between pots and PSU. For trucks with a carbon potentiometer, the resistance of R2 should be 620 ohms.
3. Verify pot voltages, V1 and V2, following the "Pinout Matrix".
4. Verify correct version of software is in the VM and PSU.
5. Check for shorts to frame.

I would guess you have/had a wiring issue in the steerhead. I've had wires just pullout of connectors in these trucks before. Let us know if you got it solved.