Update: after testing the low and high side pressures of my a/c system using a a/c manifold gauge set, I found both pressures to be pretty low. From my research, this tends to indicate a low charge of refrigerant. Next step will be to have a good shop do a full discharge and recovery, leak test, and recharge with the correct weights of refrigerant and oil.
With my trusted shop a couple hours away, I decided to add a little refrigerant to see if the pressures recovered at all. I added nearly a 12oz can of "pure" refrigerant (not the ones with stop leak, or added oil, etc) via the manifold gauge set to bring the pressures up closer to the generic ranges I found for r134a at a given ambient temp, and I am cautiously optimistic about the results. Following this procedure, the driver and passenger temps are nearly identical... and also a good 10-15 degrees cooler than before.
I may try adding some UV leak detecting dye myself to do a little more troubleshooting before handing it off to a shop, just in case it reveals something silly like a slow leak in an under-hood hose connection. It's interesting to get a little deeper into how the a/c system works, but my curiosity is outweighed by my terror of causing damage by overcharging the system or introducing air/moisture/debris.
On the bright side, I learned that the Gap IID tool was super useful for this a/c troubleshooting:
* air blend motor position
* air distribution motor position
* a/c high side pressure
* evaporator temp
It was also helpful to quantify a/c performance changes by setting thermostat at LO, selecting face vents only on the lowest blower setting and fresh air input (not recirc), and then recording temp of each vent output (both in absolute terms and as a delta off of ambient).
I'll add more here when a shop gets their hands on the truck and tells me what's really going on... until then, thanks for the help moving this one along!