Differential Fill Plug Where Is It?

Have not tried the JB weld yet. If JB Weld does not stop leak, can tube be welded to housing from under the car with everything in place, or does the whole assembly have to be taken out of the car? I am sure the leak is coming from where the axle tube goes in to the housing on the passenger side because I have a drip pan under the car and I have been under the car a few times looking at where the oil drips are coming from.