I made one by gutting an old spark plug.
Ran a 3/8-16 tap through the body.
Rounded the end of a 3/8 bolt to contact the piston.
You might try driving the car in 2nd gear and bringing it up to the same rpm you see in 3rd at 60 mph. If the vibe happens I'd suspect the engine/trans. If no vibe then something in drivetrain after the trans.
