When you say everything forward of the trans does that include the engine? Seriously what have you changed? It can't be in the transmission based on what you've said. It could be the pressure plate,flywheel or in the engine. The harmonic balancer could have spun , I have never seen it happen in a Z but there's always a first time. Did it just start doing it or has it been doing it for a while and getting worse? Where do you feel the vibration, the steering wheel, stick shift, gas pedal or throughout the whole car? Is it making any noise?