how bad is the leak? if it just looks damp...i wouldn't worry too much about it... does it leave drippings or a puddle when you park it overnight, or is it slinging oil on everything underneath? most mechanical, oil filled components leak to some extent... and many dishonest mechanics have reaped the benifits of these unnecessary repairs...
I even had the good fortune of having the entire front axle assembly replaced on a 1989 ford ranger, when i took it in for a wiring recall, and the shop noticed a little oil seeping from the front diff... the truck had ~2,000 miles left on the warranty, and the shop was very slow, so they replaced every seal and bearing on the axle, under warranty
