If the vibration goes away when you isolate to a single variable (Park/Neutral vs Drive), then whatever the last variable you changed was the issue.
Brown Dog or not, you already gave the answer earlier in the thread. When you are in drive the RPM's drop. But when you put your foot on the gas to bring them back up to the same RPM when in Park/Neutral they go away. ONLY in that scenario do you have a valid, 1 variable comparison.
The problem is that your engine is idling low when in Drive and the 6 is out of balance at that point. Can the bushing solves that problem? Maybe, but it depends on how bad the vibration is. I have the same thing in my 2006 manual - my idle is low enough that I get a vibration. Bump the gas and it goes away. Clearly not the transmission since I'm stopped. So must be the engine, just like yours. Could be the bushings, but yeah, that's a pain in the ass.
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