An update!
So I took off every ground I could find (some stock, and some added), cleaned them and replaced the ones that were slightly shoddy. Traced wires via wiring-diagram in the Bentley and made sure everything was in order, had to replace 2 that were broken in the loom (a royal PITA, there was SO much electrical tape!). I now have nearly all electrical components in good working order. Running lights work again! All blinkers, flashers, and taillights work, though my reverse-light switch is shot. I've got a new one on order. I put in LED headlights (Thanks for the tip, Libby!); not the same ones you linked, but very similar. Took about 10 minutes total to swap out both. HUGE improvement in both voltage stability and night-vision! I'm still going to dig into the wiring harness section driving the front-end lights; I have no power going to the inner set of round lights (fog lights? I don't know what these are for), but I have some aftermarket fog lights mounted under the front bumper. I'm on nasty dirt roads all the time and these low-hanging lights are.... no longer in working condition. So I'll remove those fog lights, rewire the foglight switch to run the round ones, aim them, and replace/shorten wiring to the rest of the front end lights.
Now its on to the minutia; I'm having a strange problem with the radio that I haven't spent much time on yet. I'm imagining there's yet another grounding issue.
Problem is, radio works fine until I start charging something via USB in the lighter socket; I get nothing but static when something is charging. I replaced the lighter socket (Thanks, ORCoaster!!!), and the radio is aftermarket, so I'm not too sure what's going on. I also have a bluetooth receiver that I can plug in, one of those ones that outputs to a frequency I can pick up on the radio. I get a high-pitched whine that gets louder/higher as RPM increases; I'm imagining this is a signal interference from the alternator. I get the same whining with the same receiver in my 2003 Tahoe, but not the static issue.
I bought the truck with a busted taillight... Any ideas on how to replace both units without spending $300-400? ie, does another vehicle (old gm, old toyota, etc.) happen to have similar geometry I can modify? Or perhaps theres a source I can't find that's got just the red/yellow/clear "glass" replacements?
Side note- the truck is presently covered in snow, it's 14F outside, and it's idling smoothly =)