Well, another update, late but an update nonetheless. It drove fine on my weekend test drives. Drove it to work last Monday morning with zero issues. Started it up after work, put in drive, made it 20 or so feet and speedo dropped to zero and motor revved up. I let off the gas for a second thinking "Oh $&#*!" and speedo picked back up and drove all the way home normal.
So, instead of letting it happen again far from home like the first time, I went back to the drawing board reading and reading more in the 4x4 forums. Ignition switch seemed to be the next most common fix when VSS wasn't the culprit. I've never changed one and the VERY few videos online didn't show much or explain much. Anyhow, I dug into it and 5 hours later working with a headlamp I use for hunting, I almost had the old one out lmao. Yeah, I was not going fast because I didn't want to tear anything else up while I was at it. There's not much room to maneuver and torx/reverse torx tools needed. O'Reillys had the AC Delco switch for $210+tx but I figured I wasn't needing it immediately so Amazon fixed me up for $115 and picked up a GM ignition switch torx socket as well. I finally got it put back together tonight and drove city and interstate totalling about 20 miles to test it. I really hope this was the problem and don't even care that I probably replaced a good VSS, keeps me from having to do it later I guess.
The only thing goofy now is the voltmeter seems to drop some randomly. I don't think the battery being unplugged for a week would have anything to do with that? But also don't think new ignition switch would have caused it either. Maybe just coincidence or I fixed one problem, exposing a short somewhere else, not sure. I'll probably look up where all the grounds are located on this model truck and check them.
Anyhow, keeping y'all updated and will let you know how it goes tomorrow.