ok, sounds like rear axle pinion seal. and its very easy. remove the straps holding the u joint to the axle yoke. a pry bar will pry the d shaft away from the axle. it slips into the t case without being removed.
remove the big nut holding the yoke on the pinion gear. tap the yoke off the pinion gear with a hammer, pop old seal out, pop new one in.
most important thing is this nut and reinstalling the yoke. put the yoke back on, and tighten the nut. the nut is technically one time use only, get a new one from the dealer, or here's what i do: simply use some blue threadlocker and retighten. it is important to make it tight, but do not overtighten and change the preload on the bearing. a strong impact is fine. tighten it back up, and rap it about 4 times after its tight. do not simply reuse the old nut without threadlocker, it has a good chance of coming loose.
most important thing is the nut