Okay so updates. In addition to the vibration, recently there's been a big clunk up the rear end under acceleration. I had a dig around, and the upper torque rod was LITERALLY HANGING BY A THREAD. The only reason the bolt hadn't left the assembly altogether is because the rubber bushing was gripping onto it. Good guy bushing.
So I put it back in, smothered it in threadlocker, tightened it to infinity Nm, and the clunk is still there.
I did one of those weird things where kickdown engages moments after the car shifts into second, and it made the most ungodly clunk and the acceleration kicked me in the face bigtime.
So yeah at this point I'm not sure. The bushings at the rear, while old, don't look worn out. Nothing looks bent. Shrug what do? Is it still just the prop shaft/gb extension housing bush?