dirt or asphalt ?
manual or auto?
Is there any vibration with engine at idle when the car is NOT moving?
Have you eliminated any sort of electrical miss ? plugs / wires / dist cap
Is there vibration when the engine is revved up when car is not moving ?
If vibration is present ONLY with car in motion,
- at what speed,
- under load,
- gliding with clutch in or in neutral ?