Good news and annoying news.
Good news, locking diff is installed. I didnt take many photos on this since I was just trying to push through to the end. The main harness that connectors the RDCM to the diff routes through the gland that the other harnessing passes from interior to exterior, which was easy. I tied the new harness to the old harness and just followed the old harness path, which routes directly above the diff. I then measured, cut, and terminated the harness.
At this point, install new diff and hook up connectors. I recommend hooking up the connectors right after the diff is in place. The motor connector(s) is hard to reach as it is and reaching it with the axles and exhaust in the way would be a PITA. Don't forget to also connector the diff breather.
Next I spliced the RDCU into the CAN network. The CAN network is broken into two parts, High Speed and Medium Speed. You want the High Speed CAN, which is two, 20ga wires (yellow w/black stripe, yellow w/brown stripe). There is nothing special about splicing into it. As long as you're splicing into those two wires, everything will talk to each other. I opened up the main harness at the C-Pillar and spiced in.
Lastly, you need a IIDTool or similar to configure the vehicle.
This is just for the GAP IIDTool. Others may be different. For the IIDTool, you want to find the ECU
Menu > IIDTool Config > Find ECUs.
Once the ECU is found, you can turn it on in the config file
Menu > CCF > IPC-Instrument Pack > Locking Rear Axle Differential > change from Not Fitted to Fitted
This is where the annoying part happened/is happening. Changing the diff locker to fitted immediately throws an fault, P0806, from the RDCU. This fault unfortunately also shuts off the terrain response, ABS, stability control, and a handfull of other features.
Looking this up in the FSM...
So the fault appears to be an issue with the locking actuator. The position of the actuator is different then the vehicle expects it to be. Step one is to calibrate the locker. This isnt totally unexpected, since all the FMS procedures say that if you replace the diff, the locking actuator, or the RDCU, the system needs to be calibrated. I contacted GAP and they sent me a beta build for the IIDT which calibrates the system. Unfortunately, still no dice. I still get the same fault. GAP is looking at the fault logs and the ECU data and will get back to me if there is anything else to try, otherwise according to the FSM the actuator is bad, which would mean pulling the diff all over again
So TBD on that.
At least on the good news I can pull the fuses which power the RDCU, and the vehicle then ignores the entire system like its not even there, and drives just as it did before. No systems are shut off and everything is happy. So far now, its just a waiting game to determine the next steps.