You can tell if its a locker 1 of 2 ways really easy. If there is a tag on the back with a line on it that says something like 3L 73 or 3L 55 then its a locker and requires an addative. If there is no tag you can take the back off and on the ring gear carrier will be 2 springs that look like an S laying on its side on on each side of the center chunk. That spring holds the clutches in place.

 

i thought you said it was leaking if is i would get it serviced ford does it for $100 or your self but yeah they dont use gaskets any more but i just had them doit becouse i had it done and then it leaked so ford did it twice . use the ford slip fluid or look in to royal purple maybe for the heck of it. do it right once