It's a fairly straightforward process that does not require getting inside the axle. Remove the pinion nut, then the yoke which pulls straight off... a little persuasion may be required from a large screwdriver used as a lever/prybar.
Underneath is the pinion seal that is leaking. Getting the old seal out is not easy but it should only take 5-10 minutes. I do a cut into the seal with a small chisel or flat bladed screwdriver and then start pulling it apart with vise grips and pliers. It is made from lead and rubber so it's not hard to pull apart... kind of like peeling an orange.
Be careful not to nick or scratch the surface the seal seats against.
Once the old seal is out, clean up the seal seating surface with brake cleaner or just wipe it out real good so there is no debris to cause the new seal to leak.
The new seal is carefully tapped into place once you get it started all the way around. Getting all 360 degrees of the seal started and partially seated before the final seating of the seal takes a little doing, it will want to cock to one side. Have patience, it will eventually finally seat all the way around at which point you can then carefully seat it all the way with light taps all the way around until it bottoms out & is in all the way.
Be gentle, you want to avoid damaging the new seal so it would leak like the old one.
A good seal is from SKF which bought out the old Chicago Rawhide line of seals. I recently ordered one for my D44 from RockAuto Auto Parts
who seems to have about the best prices and fastest delivery for such items. Their website makes it pretty easy to find what you need.