1963-1979 Corvette Urethane Upper and Lower Differential Carrier Cushion Kit, takes that clunking noise out when you shift from park to drive. Urethane will help keep the nose of the rear end in it's correct position for years, where rubber usually compresses after time causing an angle change.
Learn more from the following Mid America Motorworks Knowledge Library article:
Corvette and Why to Use Urethane / Polyurethane for Your Suspension