Besides the fact that I have a 3G, what info is relevant to this thread that's not in my profile?
I couldn't begin to guess.
Without KNOWING what all your mods are, I can't determine which of them are relevant to this thread.
That's why, if you're gonna ask questions about your truck, it's important to keep an up-to-date list of mods in your signature.
As far as I know there is nothing wrong with my ground to the block.
Then you need to check it. Either there IS something wrong with it, or there ISN'T. Only you
can find out for sure, so find out.
Not sure what you mean by electrical devices mounted to the body? No winch...
Well, I'm glad to hear you haven't mounted a winch to the body!
Amp? Inverter? OBA compressor? Anything that uses a significant amount of current (like >10A) and is grounded to the sheet metal.
...I do have a pair of Hella 500 off lights in the front and a pair of 50 watt lights in the back, both of which are mounted to relays.
They're not "mounted to relays"; they may be powered through relays, but the issue is where they're grounded. Are they grounded to the frame (bumper/grille guard) or to the body?
50W @12-14VDC is ~4A, so not much of a load unless there are several.
How does one determine if the body is "thoroughly" grounded to the battery?
Since the issue is a voltage flutter, the best way to check is by measuring voltage between the body (specifically, the cowl since that's where the volt gauge grounds) & the battery (-) post WHILE
there's a heavy electrical load on the body. This is slightly older than your truck, but it should be pretty close:
Note the grounds labelled "BODY"; specifically those on the body TUB (those on the L fender, hood, or core support aren't as important). The gauges ground to the R kick; the blower, cigar lighter, power point, t/g window motor, & door window motors are the heavier loads grounded to the L kick. So operate as many of those loads simultaneously as possible to put a big load on the body ground, and see if there's any voltage between the body tub & the battery ground. If there is, you have high resistance between the tub & the R fender, or the black wire to G101 (labelled in green in this pic) is broken.