If you have vibration from a mount..it'll be in gear, especially sitting still.
I've been talking to Steve, Turbo Tim and the other guy and they all think I'm crazy. They said they've never experienced what im experiencing right now. I just don't remember it feeling like this and I could've swore that if anything I should feel a lil more vibration at idle in park and smooth out as I drive but mines does the total opposite. You have a link to the tranny spacers? Maybe they may help mask the vibration
You have a standard hydraulic lower motor mount? Might be time to level out the car with that poly lower instead. Vibration at park and idle sure but after that you shouldn't have issues. I'm in a different ball park though LS4's don't make vibrations at idle like 3800's doPoly motor mount may help even things out with the poly tranny mount but then it'll be 10x more vibration. Ok I exagerated but it will add more vibration. The motor mount is actually fine, its the tranny mount that allows most of the movement. I think the bolts are straight through the mount so that probably explains why those bolts are grounding out.