Man, am I embarassed.

It was a u-bolt on the exhaust just before the cat. It needed about 3/4 of a turn for the rattling to stop. I cannot get over how much noise that single bolt created.

And now the sweet, sweet purr of a rattle-free 300SDL.
http://home.comcast.net/~grindmarc/merc_fixed.MP3

Thanks for the help everyone! I think I spent about 4x more time posting this thread than it took me to fix the problem!