[3.4 grape of wrath]

I feel your frustration and anger. I had a vibration problem a couple years back and kept a diary of things that I checked and replaced. Similar to what you are going through. It ended up being the exhaust system. The combination of the rubber hangers hardening and the alignment was off from lowering the car. The noise would change when I shifted from drive to reverse. The way I discovered it was tapping the pipes with a rubber mallet from front to back with the suspension loaded and heard this loud vibration towards the rear.
I replaced all the rubber hangars and realigned everything and the vibration was gone.