Got her running! Ran the truck for a few minutes, shut it down, cranked again, ran for a little. I repeated this several times just to make sure that it wasn't a fluke. Thanks to everyone who responded and all suggestions received. Having a community like this one definitely helps keep me sane and confident when working through issues on the Runner. One of the main reasons I will continue owning one as long as I'm kickin'.

Here was my process:

What got me on the right track was running new grounds to chassis and upper intake plenum. I had a spool of thick 2AWG welding cable and lugs, so I made up some temporary cables to rule out bad grounding. Ran them from the battery negative terminal to a bolted spot on the upper intake as well as a bare spot on the chassis. After doing this, I was still experiencing a single click when the key was turned to start, but accessory power would NOT drop out.

Side story about my grounding issues: When my stock alternator died on me months ago, I replaced it with a CS-144 alternator from the 1995 Cadillac Deville (search for "Doc's CS144 alternator upgrade" for details). While doing the replacement, I removed the stock engine ground cable as it was gunked up and nasty. I ran a new ground from the alternator to the battery and from battery to the chassis. Excited to test the new alternator, I cranked her up and she ran like a dream- my alternator issues were gone. Even WITHOUT a new engine ground, the combination of alternator ground and chassis ground had me running good for MONTHS until now. It is interesting that grounding seems to have surfaced as an issue only now. I wonder if it may be the cold, or a higher amp draw.

To get the 4Runner cranking, I ran another new 2AWG cable from battery positive to the starter. The original was nasty and gunked up, so I figured a replacement was in order either way. After running this new starter cable, she fires right up.

I'm leaving this here in detail in case someone comes across the same symptoms. The following would all be on the top of my list to check for anyone with these symptoms:

Battery health. Try a jump-start, try swapping in a known good battery.
Fuses. Check your main ALT fuse, your AM1 and AM2 fuses in the engine bay fuse box and make sure they aren't blown.
Ignition switch. Your ignition switch should be passing 12V through it's main connector whenever you turn key positions to the starter and accessories
Main wiring/Grounds. Turns out this was my issue- I did not have quality grounds and the electrical pathway from the battery to the starter and back simply was not good. In my case, there were no intermittent issues as warning signs.