It's right at the tail of the transmission, at the front of the driveshaft. There are exhaust pipes below mine, but it's still easily visible from underneath the car.

It looks like this:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1WW15C3EP1WW15CDOW&year=1997&make=MB&model=E-420-002&category=K&part=Drive+Shaft+Flex+Joint