You might be able to get the rest of the pin out with a strong magnet and a steel pick. You'll need to get the pick in contact with the remainder of the pin, and magnetize it. I won't quarantee the pin will come out, but I have done that trick in the past. I have also heard of guys using a coat hanger and a stick welder to get the pin out. The hanger is bent to access the end of the pin, then is clamped in the + cable from the welder. Hit the power to tack the hanger to the pin and pull it out.
This is actually a pretty common problem, it usually happens when the diff is assembled at the factory.
