Chrysler Repair: Question of diagnosis for Chrysler Sebring, chrysler sebring, belt tensioner

Question
I have a 2001 Chrysler Sebring, sedan, 4 cylinder overhead cam.  It has had a problem diagnosed as a faulty belt tensioner (serpentine) which I replaced last week.  The problem is that when it is at idle, perhaps while stopped at a  stop light, the car shakes.  At first I noticed it only doing it when the ac was running, now it does it even without.  The replaced tensioner has not fixed the problem.    Do you have an idea of what the problem is?  I am thinking but hoping not that it is the transmission.  Thanks!

Answer
alot of times if the engine mount on the upper right side is not centered properly it can cause a vibration.However it could be a lot things.And not being able to feel it,I would just be guessing.Has been tuned up?Do you have a check engine light on?When the ac is on it puts an extra load on an engine.If it isn't running well to begin with,having the ac on will make it run worse.