Vibration at idle
 

A buddy of mines wife has a 94 camaro,V-6,auto,bone stock.Stopped at a red light,with the car in drive,the whole car vibrates pretty bad.You can put it in park and it quits.Car drives fine the rest of the time.He thought it was a pulley on the water pump out of round so he took the belt off and the vibration is gone,put the belt on and it comes back.He's changed the water pump,belt tension pulley,altenator, AC pulleys and the belt.Didn't help anything.Anybody have any idea what it might be?
Thanks in advance

If it only happens in drive, I would suspect its transmission shudder due to inadequate fluid level.

My car sounded real bad at idle, like a miss. It was due to a bad EGR valve. A light rap with a mallet reset the float on it and it was good to go.