Thank you for leading me down the right path!
I'll type up what I went through for others who may encounter this thread:
After ordering the fuel pressure gauge, I decided to proceed with more research and other fuel system troubleshooting steps while waiting for the gauge to arrive.
One step included testing for power to the fuel pump.
I disconnected the 7-wire electrical connector that goes to the fuel tank (located under the car, not far from the rear-left tire) and connected my multimeter to the wires that power the fuel pump. The pink/black wire is positive and the solid black wire is negative.
I turned the key on but didn't get 12V.
This led to many hours of testing & researching and a final conclusion that the PATS (Passive Anti-Theft System) was likely to blame.
I had never heard of PATS before and thought the flashing THEFT light (on the instrument panel) was just a glitch caused by a failed sensor, or maybe related to having my alarm system removed a while back.
Further troubleshooting revealed that it was likely the PATS transceiver (the ring/halo) that was failing.
At this point, I had learned a lot about PATS and just wanted it gone from my truck forever.
After more research I found someone who could make changes to the PCM which strip PATS of its power for $60 + return shipping (Brian at EFI Unlimited ... his name came up pretty often while searching forums related to car repair & tuning), so I removed my PCM (was more of a pain than I thought it would be), packed it up and shipped it out.
I wasn't 100% convinced that PATS was the problem (I just wanted it gone either way), so I resumed troubleshooting steps while waiting for the PCM to arrive.
One of the steps included testing for a dead fuel pump.
I disconnected the fuel filter and put the fuel line (coming from the gas tank) into an empty container to catch fuel.
I disconnected the 7-wire electrical connector again (same one mentioned above) and connected my battery directly to the wires that go to the fuel pump. Same colors as mentioned above (pink/black = positive, solid black = negative). I took my battery out and brought it over by the rear-left tire to do this. I also added a temporary 20 amp fuse between the pink/black wire and the battery (using alligator clips), just to be safe.
I could hear the pump buzzing lightly but no fuel was pumping into the empty container.
I tried a few more times and still no fuel, then the pump went silent.
I pulled the fuel pump out of the tank (huge pain, but I followed someone's instructions online for removing the pump without removing the fuel tank and, with a little help from a friend who crawled under the truck with me, got it done).
I connected the battery directly to the pump and it was indeed dead.
I bought a new pump for $98 at a local auto parts store (just the pump with some fittings - not the whole pump assembly), installed it (another pain ... had to get help from friend again), tested it like I tested the old pump (battery connected to the pink/black wire and solid black wires) and it worked; it pumped fuel into the empty container I set up.
I put my battery back in, reconnected the the 7-wire electrical connector, reconnected the fuel filter lines (with a new filter of course) and continued to do whatever troubleshooting I could while waiting for the PCM. I found no other problems.
When the PCM arrived, I installed it (was easier to put back in than to take out because I knew how to angle it), primed the fuel system by turning the key to the ON position a few times, tried starting the engine and it fired right up!
Took it for a test drive and did about a dozen more starts to make sure everything was solid.
Only problem now is that the THEFT light on the dash is still flashing. It's the 1-6 flashing pattern (1 flash followed by 6 flashes, repeating over and over).
This is apparently common after relieving PATS of its power. I'm just going to open the dash and add a switch for the light so that I can disable it while still having the option of re-enabling it if I want/need to.
So... yeah, it's really great to have my truck running again.
Thanks again to stamp11127 for directing me toward the fuel system.
I'm guessing the fuel pump was the primary problem that was causing me to have to wait a certain amount of time before the engine would restart, and the PATS problem surfaced more recently, completely paralyzing the truck.
I don't know what exactly caused the PATS transceiver to fail. Maybe just time.
