I wanted to post a follow up after the repairs.

I replaced the PCM (ECM) and the voltage immediately popped to 13.6v. Fixed!

I had a few instances since, though , in which the truck would not start. I checked the simple things first - and found the culpret: battery terminals had a layer of black oxidation on them. Scrubbed them clean, and added some battery terminal grease - and all is well with the bee-mobile now.