Leaking Diff , what to do?

ive noticed a small leak over the last coupe of weeks from the diff of my 67 new yorker, over the 2 or 3 weeks its leaked about a palm sized patch of oil, its leaking from where the shaft meets the diff , there is a car show and cruise this week , will my car be ok to drive in that or is this a major i must fix asap, if so, what should i replace?
Author: peterlucas