I use Fel-Pro myself, part #RDS 4304. Before you take the cover off, can you get the ID # that's stamped on the front of the differential carrier? It should be at the top, on the front face of the casting. From that, the gear can be determined, or at least what should be in it. If it's a '55 (2nd) axle,the code should be AF, which is a 3.90 gear.
