the flywheels are balanced reasonably well at factory to each crankshaft..
It's possible the flywheel is out of balance but it would have to be majorly bad to cause that vibration.. I had a resonance vibration at 3.5 to 4k and it was my alternator tensioner bar was completely loose. Does the vibration do it with the clutch cable pushed down,, worse or better with it down.
Im not talkign about the release bearing but Just maybe the clutch isn't aligned properly.. When you drive the car is it smooth or not?
Do the simple things before u commit to an engine out..
could be something simple... but to feeel it through the whole car !!!!
yikess