Since I've been suspecting the aux fan I researched a few posts RE related issues. A few people recommended unplugging the AUX fan, starting the truck and then starting the AC - in order to check two things: 1.) Does the compressor clutch engage, and 2.) does the AUX fan show signs of life when it's plugged back in (obviously with the truck + AC still on).

So I did just that and voila - with the aux fan unplugged the compressor clutch fired right up. I checked the truck so see if the air was actually being cooled and it was. I decided to check the psi of the low side line since the clutch was engaged and I received a more appropriate reading of 50 psi (given the conditions). After that, I plugged the aux fan back in and it attempted to rotate a few times for a couple minutes and then stopped trying. It was about that same time the air inside the cabin was no longer being cooled.

Of course, I wanted to rule out anything electrical so I tested the aux fan connection for voltage and was happy to note a ballpark range of 12v.

Long story short - I'm about to order an AUX fan replacement (64546921381) but not before acting on upallnight's recommendation of getting a scan. I'll check back in with results.