Got my new Motorcraft thermostat installed (I've decided I'm not doing any more mechanical projects under the hood or under the vehicle, except changing my oil/filter. I just don't have the temperment for it!). I'm happy to report that my temperature gauge now goes to exactly where it should - nearly halfway up the gauge - and it stays there! Never budges.
Here's a before & after pic:
I tossed the old coolant & refilled with Zerex Gold. The levels of coolant in the reservoir kept varying a lot after refilling & driving around, it seemed like I was losing coolant somehow. I noticed milky splotches on top of my coolant-reservoir...So I bought a new plastic coolant-reservoir cap today at Advance Auto, and I screwed it on very TIGHT after the truck was cool. I drove it and did some errands, and it seems like the coolant level is now staying consistent and no more coolant puking. And again, the temp gauge is working just as it should now, so I'm hoping I can finally forget about cooling issues, radiators, etc. from now on.
Thank you so much to Soledad and everyone who helped me in this thread.