91 Z-28 5.7 .....vibration at 3500 in my driveway in park. Checked the balancer appears to be ok...any ideals ? Can the torque convertor cause this ? Also vibration comes in @ about eighty miles per hour on the highway at about 2700 rpms..driveshaft ahs been balanced as has the wheels..Still searching ?

 

bad motor mounts and that is just the engine speed that creates the right frequency to make it noticable?

since it's in park it can't be anything behind the tranny right.

flex plate broken?

 

 

This is simple to check. I would raise the car, remove the flexplate shield and shine a flashlight. Rotate the engine and you can see the entire flexplate. Turn it via the crankshaft bolt. If that is fine, have you checked the harmonic balancer? You would need to set it to TDC for #1 and see if it is at 0. Your balancer could have come off the rubber and rotated out of place.