After plunger, draino, simple green, and another industrial strength degreaser, I finally dealt the killing blow to our backed up drain!
On the outside of the house, matching the part of the wall where the sink is, there was a plug. I unscrewed the cap, and inplugged it. A bunch of water (from the sink) came rushing out. About 1 inch inside the pipe i’d just opened up there was an intersecting pipe leading down. I got my cordless drill and my auger attachment. And after all 15′ (mine may have been 25′) of auger I finally unclogged my drain… pulled out some gunk that mush have gone down the disposal by accident, and theres some real nasty grease down there too… but it’s flowing again! YAY.