So it turns out that it wasn't my rear diff at all, thank God. My e-brake shoes and pads came loose and got jammed up between the regular pads and the rotor. I don't know which side it was on, but I guess that explains why it was a lot worse when I was backing up than it was when I was moving forward. $350 for new shoes and rotors (cuz my old ones were pretty nasty).
Better than a poke in the eye with a sharp stick.
I still owe you chaps a vid of my exhaust. I've been slacking on that.