Best guess on my issue ?

I'd say driveline......have it checked out. But.....does it vibrate @ 3500?.......or is @ a certain SPEED? If you put your car in "N" and find a nice long downhill to get your car coasting up to 40 MPH (or whatever speed it vibrates at) that should REALLY pinpoint it to the driveline. If it's a "3500 RPM" thing, then it could be more in your engine balancing.