my 1998 e320 always vibrates when the car is in a stop and the transmission is in drive, now my 2000 e320 is also doing the same very thing. what could it be, it has to be something related to the transmission because when i put the stick in the parking position the vibration stops.

sounds like motor mounts homie.

how many are they and where are located? could i buy them in a dealer and do the repair at another workshop? or you recomend me go to the dealer?

Could be idle speed related. What is the idle speed in Park and in D (presumably when hot)
If different, does the vibration go if you increase idle speed in D up to the same speed as it runs at in P? TAKE CARE! CAR MAY START TO MOVE FORWARD IF YOU INCREASE REVS TOO MUCH.
Could be a misfire at low revs. Any fault codes. (Need an OBD reader.

Stuart

I'd say dealer because usually, aftermarket mounts tend to crack a lot faster and are of poorer quality. There are two I think.

I agree with Battlecat get yourself OE only. THey cost more but last longer. Not an easy job have your mechanic do it. By the ways there are 3 of those mount. OFten overlook is transmission mount.