A word of advice. Before you try to take it out make sure that you clean the hole that the 3/8" drive goes in . It usually gets filled up with gunk and if you don't clean it out you can easy bust your knuckles on the underside of the car. And mine was really tight.

I wish FORD would have put the fill and drain plug on the diff cover. Very silly engineers.
I think it was for a structural reason with the cover. Plugs weaken its integrity, so a much heavier duty cover (almost like that of the aftermarket girdles) is needed. That'd be a big cost outlay per vehicle.
Guys have tapped the pans before but it's not something I'd do.
How the hell are you supposed to even fill the damn thing then?? When I did my t-lock and gears I used 3/8 inch vacuum tubing and a hose clamp to secure it to the funnel on the fluid bottle and inserted the tube in the fill hole. Somehow I managed to get most of the fluid in. And also what is the capacity of the case?
Wise advice here. Most extensions these days are wobbles, and they suck for this application. Also, wear mechanics gloves, as more than one fellow stanger has ripped a nice chunk out of his hand when that plug lets go.