Check and see if the pinion seal is leaking. I've also seen where a rear main seal leaks and ends up on the differential (my xj did this). If it's not the pinion, I'd clean it off and keep an eye to pinpoint where it's coming from. Good idea checking the vent tube, just to be sure, but have never seen one come off before.
The beatings will continue until attitudes improve!
HID Quad bi-xenon + foglight retrofit