This may be an easy fix.  Imagine that a previous owner had picked up a
piece of fishing line, that tore up the front pinion seal, or that the seal
just wore out.  That would be pretty easy.  On the other hand, imagine that
the leaky seal is a sign, that the front pinion bearings are worn out.  That
would be a more expensive fix.  Get under there, grab the front drive shaft,
and figure out if the part that goes into the differential housing, has any
play in it.

It's probably nothing but a leaky seal, but you never know.

Earle