Check this link.
http://www.differentials.com/install.html
Replacing the pinion seal requires some special tools and skill.
If you can not find where it goes and need it done fast it might be better to let a repair shop do it for you.
Or if you can get the tools needed and a good repair manual we can try to help you with it.
First thing what rear end.
Is the yoke lose.
How many miles on it?
Is the rear end making any noise?
Always check for what may have made the seal leak.
How low is the rear end on grease?
Is the vent stopped up?
MT