rear diff seal

[James Russell]

I just had a leaky left rear differential seal replaced, by the dealer.
Drove around a little, so I take a peek under the car, and there is a nice
little black puddle under my rear diff. I will take it into the dealer and
point it out and make a little stink.  

My question is what would cause a diff seal to leak. Time? Locking the diff
and trying to turn? Not seated right?  

Thanks

Brad Russell
84 4ksq ~199k