check for loose nuts on your studs. I had wicked vibration the other day, couldn't figure it out and when I started to mess with the muffler noticed that one of my studs on the engine mount had vibrated loose. I loc-tited it and tightened it up, much less vibration.
go to lowes or homedepot and get locking nuts, and or double nut your stubs so they don't vibrate loose. i have lock nuts and double nuts on my front mount, thats the mount i worry about the most as it looks a bit too flimsy. You might also want to get higher grade studs if you have not done so already.