Mine was leaking from the cover as well. Go buy a 12 dollar gasket at advanced auto get yourself a socket and undo all the bolts on the rear end let it drain into a pan. clean where the gasket goes. I'd also check the guts to see if there are any wear on your gears. Put the gasket on, put the bolts back in. Tighten them up, Take your plug out of the front (all you need is an extension) You can reuse the oil from your pan. It won't hurt nothing. Or just get a bottle of rear end oil. To me it doesn't matter what kind it is.

I had to do that to my 95. Your mechanic was trying to screw you. It's a pretty simple job.

 

Double post FTL

 

Actually, they do have a vent. Thats what holds the distribution block for the brake lines.
You'll need friction modifier as well as gear oil. Trac loc in all the mustangs.