You said that there's no vibration in park, or neutral. That says the vibration is in the driveline.
Not necessarily. You can get a vibration when the drivetrain is under load, which you don't have in neutral. The torque of the engine when in gear and under load transfers that torque to the whole drivetrain. Anything on the drivetrain that is close to the body or frame can move close enough to hit during acceleration (or deceleration).
