Rev it sitting still with the clutch pedal depressed. If it vibrates is a balancing problem with the motor, likely the balancer. If not then engage the clutch with the car in neutral and rev it again. If it vibrates now you have an issue with the input shaft on the trans, potentially the pilot bearing. If it only happens when you are moving its a driveline problem, probably the driveshaft/u joints but it could be a mount problem on the rear or the motor or the trans mount as well. My car vibrates like crazy right now and I'm doing the motor mounts tomorrow morning. The passenger side primary is damn near hitting the K, that is also something to look out for especially on an older car.
Also could be as simple as a wheel/tire balance.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.