Check for a warped axle flange. Sounds wonkey, I know, but since we are trying to rule out all possible issues.
maybe one of your rear disc's is warped
I worked on a crazy vibrator when i was employed at the ford stealership. I ended up unbolting the exaughst, trans, cross member, rear u bolts, and motor mounts. Took a pry bar and kinda wiggled everything around, then bolted it all back up. And it worked! But this was on a expedition. Also I didnt take the bolts out. I just loosened them up enough to move.