1997 Differential

My 1997 Explorer is leaking fluid from the rear differential from around the pinion gear. I looked this morning as to where to add the gear oil but now I am stumped. It is a 2x4 4.0 litre. Can I just add gear oil as it is not leaking much. WHERE THE HEAD IS THE FILL HOLE??? If I want to go ahead and change the seal how bad of a job is that?? 125000 miles and the dang thing will not die.
I really want my '05 F250!

There is a fill plug on the side
of the differential caseing.
The oil is synthetic, and is expensive.
It has to be pumped in.
The seal should be replaced....not a big job.