Please can you describe as much as you can about the problem.
When did it start?
Does it happen at idle?
Does reving the engine while stationary produce the vibration?
Is there any loss of performance?
Is there any visible exhaust smoke?
Are there any noises, other than the vibration?
Any warning lights?
Anything else you might have noticed.
There could be many reasons why you have this problem so as much information as possible to help us help you.