So I opened the diff to find what one would only hope to find on a vehicle that hasn't been trashed or seriously neglected: A perfectly good diff with normal metallic shavings in darkish yet still amber hued fluid, at least when drizzling through the light it was. I banged around on the exhaust again after I changed out the fluid in the diff and wiggled, twisted, and shook every bit of drivetrain stuff I could get my hands on. I have no play, no slop, no leaks, no rattles.
This leads me to my next suspect:
Transmission. I'm curious about the possibility of this being in the torque converter since it only happens on take off and then fades out, unless I ease into the throttle gently up to about 10 mph or so, in which case I can avert the noise all together.
I will say that I must either be a complete idiot with the questions I am posting here or the world has mostly left me behind. Seems like folks posting in the "technical" forum about which seat covers are the best color match, or whether to get an antenna in chrome, satin, or black, or which tires "look the most aggressive" etc., get a LOT more responses and input than questions I am asking about mechanical parts I have very little knowledge about. Luckily for me I have fairly thick skin and I have to get the information sometime, somewhere, from somebody so I'll just keep searching and asking. Have a nice day.
Swapping out every part on a Jeep, one at a time.