I'd say a drain and fill with any good quality 75W90 or 80W90 would make your diff happy. You can bet your mortgage that it's never been changed before! Get it warm as you can before draining (10 miles minimum), as the stuff in there will be like sludge. I like synthetic lubes everywhere I can put 'em, but the diff is possibly the least necessary. Synthetic will, however, make a noticeable difference in really cold weather - when I changed (Redline in my case) I noted the car was much less "stiff" on winter mornings.