Losing Diff. Fluid Thru Tube

Recently I replaced the front axle u-joints at the wheels. Passenger side is cool, no leaks. The driver side "short side" is leaking axle "dope" out the end of the tube. I probably damaged the seal removing or installing the axle shaft. Is it difficult to replace? Can it be repaired from the outside (thru tube) or do I need to take the differential apart. Otherwise I'll just keep adding more "DOPE"

BTW it doesn't have one of those vacuume disconnect thingys (command-track?). Just a one piece axle on each side. Dana 30. Thanks in advance.