A few weeks ago I replaced the Ball Joints on my pickup. Removing the axle shafts was part of the process but now I have a gear oil leak coming out of the axle tube. Is there a seal inside the tube near the differential??

 

Yea there is a seal in the diff on both sides. You have to pull the gears out to replace them. However the oil you're seeing could have just leaked while the axle was out and taken a while to run out of the tube. I'd try to angle a pressure washer wand in there with it assembled and rinse it out then wait a few weeks to check before tearing it all apart again.

 

You should check the vent line - if it is clooged pressure builds up in differential and forces oil through inner seals