Quick update, I got out the multimeter and tested around per the TM. Wasn't getting a solid trail to go on. Went back to the battery tests, and the batteries were testing all weird/low. Tested individual batteries, and still over 12V each. On a whim went back to starter switch and now it all works. Anyhow, I'm going to leave it alone for now and I've ordered the Kascar grounding kit as my best next course of action. Thanks for all the replies and thoughts. I'll tackle this one holistically with the add of the grounding harness and battery cutoff switch.
"Went back to starter switch and now it all works."
Meaning?
Starter switch is bad? Loose wire?
When you DO install the cutoff switch, check all the large battery wires for corrosion at the terminals. I had one that was being held on by the heat-shrink alone, as have several other people here. Copper salts will only conduct so much current - you'll get a good voltage reading - and then not enough to start the truck sometimes.
Probably a good time to "gut" the battery box, check all connections, take some light steel wool to them and add something like "Deoxit" and reassemble with a liberal slathering of dielectric grease.
This way, one component at a time, you're bringing the truck back to spec and can "trust" that system won't (or will be less likely to) fail.
Have fun.
man I wish 
