It's only my opinion but I would never just seal a leak.
I would want to know why it's leaking as it could be a symptom of something more serious. Then again it might not but I am a firm believer in root cause analysis wherever possible. (Which can be a lot of fun on a Morgan)
One possible cause could be that there is a very slight amount of flex in the diff housing to tube joint which might have started a crack around the weld and just applying sealer to that would allow the flex to continue. Of course that is the pessimists worst case scenario but it can happen.
I might be tempted to clean all the paint off in the area then use a dye penetrant crack detector to rule out that possibility.
As far as you know, is this a new leak or has it always done it?
