I'm sure its replaceable but there's a lot involved in taking a differential apart and then putting it back together again, and I'm not sure about parts availability. Then there are issues on shimming gear clearances, etc.
I went with an excellent low-mileage used differential at $135.
If you want to see pictures of the severely worn differential:
http://www.peachparts.com/shopforum/showthread.php?threadid=69421
Ken300D