Car shakes when stopped in drive.Which Mount?

My car shakes only when stopped in drive.Idle is steady at ALL times.Runs smooth in Park,Reverse,Nuetral,and when moving.It has done this since before we bought it.We knew the previous owner.I have yet to change the tranny fluid and filter, it's on the to-do list but I'm pretty sure that wouldn't cause this kinda problem.:rolleyes:
I have a 99 malibu with a 2.4L DOHC 4cyl automatic. It has a pass side mount, a front and rear driver side mount, and a braket holding the back of the trans to motor.Is there ANY way to tell,other than looking,which mount would most likely to cause this.Or should I change them all.:banghead: 60$ each ain't cheap!