From memory when I had my Transfer box out there is a small channel that runs back from the hole, it is covered by an oil seal in the housing, I would think the oil seal has moved so as not to cover the channel letting diff oil run out, I just had to reposition my seal correctly as I had removed it when I stripped the transfer box.
Thanks for the reply. After taking the diff apart. Turns out the inner bearing behind the seal has eaten itself.
And also chewed the aluminium casing.
I'm unsure if its salvageable.
I have spoken to the customer now.
If anyone has a good one for sale please pm me