Engine Vibration 3k RPM

If it’s worse in neutral, it is not the driveline or the rear axle.

check ignition timing at point of vibration

what are the motor mounts condition? All 3 ok?

Is the exhaust secure?

How about the harmonic balancer? What’s that condition and age?

If the engine received new pistons but didn’t get balanced (which is sometimes ok to do) than it may vibrate like this all the time at that rpm.