Diff oil out the vent, why?

John Trickler said

pull the fill plug out and it does not run out. The fluid is 1/4" to 1/2" below the plug. The ent is not plugged.

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That may still be over full. That's not the correct way to fill it. The capacity is listed in the factory service manual. Almost always without fail when you fill a differential using that method, it is over full.