I interpreted @Discoidiot's question as asking about an aftermarket immobilizer, because...

1. when I insert the key it releases the steering lock, and
2. I am getting an audible click from the starter relay when I turn the key to ignition

meaning I have successfully disengaged the factory immobilizer. If it is possible for those those things to happen and allow the immobilizer to remain engaged, then someone will have to break that down for me so I can troubleshoot it as well (assuming my other paths to success continue to fail)

 

As mentioned earlier, clean ground cable that goes from battery to alternator bracket, remove bolt at alternator end. You have to have a good ground to ensure correct starting. This is a known fail area.

 

A faint click is a relay. A louder clack is the starter solenoid.

If faint, then the solenoid is not working, either not getting power or open circuit or coil failed. Check by jumping 12 V directly to the solenoid (small wire), starter motor should spin. The jumper could be connected at the relay terminals mentioned above.
If loud, then either the solenoid or starter motor is defective or not grounded well (as discussed above). Check by bypassing the solenoid with a screwdriver (as discussed above), starter motor should spin. If it doesn't spin, then either motor is defective or not grounded.

 

A little delayed this AM because I forgot the Dutch F1 GP started at 0900. Haven't missed a race since 2007!

Please read in its entirety:

1. Just got back from the part store with a piece of battery cable to jimmy a jumper for the relay and a new multimeter. The packaging on the multimeter must have been made from the same plastic they use for PVC because I couldn't tear it open. Instead I just went straight to popping the hood on the Disco to attempt a start after she's been sitting overnight. Now I'm getting nothing. No relay click, no solenoid click.
2. I ran inside to find a bandsaw, opened the multimeter, and ran back out to the truck. Initial readings (which I didn't get a picture of) showed 12.1V to 12.4V depending on where I tested it. Alternator, battery terminals, battery leads.
3. The missus is out of town so I had to wait until a stranger walked by and politely asked them to turn the ignition while I read the voltage...only now the volts had dropped to 11.3V to 11.5V - what does this mean? Bad/low battery? Because remember earlier in my post/yesterday morning when I would crank I got clicks, and when I jumped her with my second car I was also getting clicks, well into the evening. All of a sudden now, no clicks?
4. The ground cable to the Alternator looks decent, but take a look at the attachments for yourself and be the judge.

I also want to test jumping the starter but I'm uncertain what touch points were being referenced. Can someone guide me on this? (photo also attached)

Starter (top of image is front of vehicle, bottom of image is read of vehicle)

Voltage reading

Alternator ground

 

Ground looks like crap, remove and clean everything, mounting point, bolt, and cable, both surfaces. 11.5 is too low, you might get starter to spin but truck likely will not start. On the starter there are two large nuts, one attaches the cable, the other attaches the starter conductor tab to the solenoid. Jumper between those two large nuts with a screwdriver. Did you jumper the relay yet?

 

1. I've got some wire brushes I'll take to the ground later this afternoon. (The Florida heat and humidity is brutal today and it's the hottest point in the afternoon - truck is currently street parked since the tow truck couldn't maneuver into the garage)
2. Once I clean the ground I'll attempt jumping the starter to see what happens
3. I did not jumper the relay yet, I'll try that after doing #1, re-testing the voltage, and trying #2. I'll probably bring the other car around as well to see if a battery jump can give the Disco an extra boost assuming the relay tests well

 

@joefred16 Yea that ground is pretty bad, and 12.1 then 11.5 is a problem. I would check the entire ground cable and all the connections There are 3 on that one and clean them all up, check the cable status as well.

 

In first photo: 12 V on the red wire terminal should produce a small spark and clack the solenoid, then the starter motor should turn. A big wire jumper from the large bolt/nut to the black starter lead (may need to remove the plastic cap) will produce a big spark, then the starter motor should turn. Be very careful not to touch the jumper to any grounded metal (the spark will be even bigger).

In the second photo, it appears the body ground tab is disconnected. There should be a ground terminal right behind the battery to connect to.

In all the photos, you can't tell by looking if the battery or ground points are corroded. Best way to tell is by disconnecting and polishing up. A very low ohm, high amperage meter could also be used to check terminals, but low cost meters will not provide enough amps to check.

I helped a motorist in a parking lot one day. He was replacing a starter solenoid. Turned out the connector at battery negative was very corroded. He was on his way after cleaning it up. Hopefully your problem is similar.

 

@JohnZo good catch on the body ground

 

I see that you didn't reply yet, so I guess you don't have it fixed yet.

11.5 is a dead battery. You definitely need to clean all of your connections, ground AND positive too. I'm leaning toward bad alternator. Clean all of those cables and charge your battery. With a fresh charge you should start. Either way, immediately get your alternator tested. You could also go ahead and pull it and take it to your LPS and have them put it in their tester.