Shifter/Car Vibrates, Why??

My car, and shifter, seem to vibrate at around 3500 rpms. While
stopped, in neutral, it will vibrate the shifter slightly. But, nothing near like it does while moving. It is not speed based, but rpm based. I looked
at the balancer in the front, it doesnt appear to have a noticeable wobble to it.

I do need to replace the throw out bearing. But there has to be something else that may be the cause of this vibrating.

Someone said maybe the springs in the clutch, but if I dump the clutch and it peels..does that rule that possibility out?

Does the motor rock too? You could have a bad motor mount causeing your tranny mount to break.

No the motor mounts are new, poly. Engine doesnt shake..
I tried again while in neutral, and it will vibrate the shifter around 4500-5000 rpms.. but the car and shifter will start to vibrate around 3500 while driving.

I havent been investigating the range in vibrate, as I recently got the car. But I pretty sure it vibrates between 3500-5500 rpms while driving.
and goes away at the higher rpms

Ok, what kind of car is it? Year? Motor? ect ect.

1990 302 lx.

Has TFS pistons, E cam, 40p heads, mac headers, Explorer pedlum/manifold, 3.55 gears, Alum DS, tremec 3550, short shift, RoH's, H-pipe.

thanks for reply