Please help me pinpoint oil leak..

[johnny1423]

Hi all,
Please see the attached photo...angle is from the passenger side looking forward at the back of the bell housing/ rear main.
On my 2000 E-320(WITH 49K),I discovered this.It is oil, not trans fluid.
It was about to drip..so I cleaned it off.
The photo was taken after I then drove 2 days/100 miles.
I appears to be coming out of the "vent" and not the actual seal in front of that.
Any Ideas???
What would/could cause a relatively new, low mile car to start this?
How serious is it?
How big of job ($$$) to fix?
Help
Thanks!!!!