Rear differential leak
 

Just looked under the car and noticed that the differential housing is wet with the lubricant. I am not sure where it is leaking from and why... Any ideas what to do next? It does not seem to be dripping, just wet all under and a little above/around the vent... Help anyone? the car has 80000 miles now...

Might want to check if there is any damage to the diff.

Probably just venting condensation and bringing oil residue up through the vent with it. I wouldn't think you have any damage (if the gears/bearings are quiet) but definately time for an oil flush and refill. Don't settle for an oil vacuum, get the rear cover removed and drain/clean properly and don't forget the friction modifier additive.

First thing to check is if the vent is open. Sometimes the vents get clogged and it will pressurize the differential. If it is open, then check as others have mentioned.

I guess I'm gonna check the vent, clean the housing with the carb cleaner and check where its seeping, plus check the level through the filling hole... However, I am disappointed... 80000 highway miles and already a leak???

A TSB has been issued for 2005 to 2010 Mustangs that leak oil from the axle vent.

TSB 10-3-5: REAR DRIVE AXLE VENT—OIL LEAK

Replaces the vent with a bladder - part number AR3Z-4A058-B.

I guess I should have been clearer - its not only wet around the vent, its wet all under the differential...

The only time I've had a diff leak in 15 years of vehicle ownership is when I ran over something and crimped the diff cover. 79' capri 5L...never noticed it...riding down the road (fortunately in a residential neighborhood at low speed) and BARK! locked up solid. Total diff rebuild, wasn't cheap.

Oil tends to migrate all over the axle. Can you tell if it's leaking from the cover? If it's leaking from the cover, it's easy to drain and remove. Then scrape off the old gasket and re-seal and re-fill with gear oil and friction modifier.

Pinion Seal?