Check your fan clutch. With engine off, see if you can wiggle the fan front to back. If this moves, this is the cause of your vibrations. My mechanic wants to order the fan and clutch. I searched online and the clutch is about $100 and the fan about $50. Problem is he wants to order them for $250 + labor. Hate negotiating but with the discrepancy, it's worth it.
Good luck!