Well, it seems that I've solved the mystery of my crazy backend. I got under the car today and was looking around. I jacked up the rear passenger side and had my brother push and pull on the side of the wheel. I saw a lot of play coming from the spindle bushing. even the end of the trailing arm was bent out a little from the spindle twisting a lot. I could only imagine how much flex it had when cornering, let alone what my brother could produce.
Here's the factory diagram and I've included a mystery arrow to show where I need the bushing, and how it's not in the diagram:
So does this mean that it comes already in the spindle? If that's the case, are there any other solutions? Could I add a washer or two to tighten things back up, or would that not work?