Had the rear pinion seal fail on my old MFL 2.5 some years ago.
GM seal was about £30 from memory and god old Daz replaced it whilst doing the autobox.It wasnt a difficult job but did make things a whole lot easier with the car in the air.Trick was to replicate the original setting of the pinion nut to the mounting to ensure the original preload was maintained,as remember reading that a members diff was shreaded quite quickly when just the seal was done without much attention to the preload.
Mark DTM,s guide was followed when job done originally with slight difference of also counting the " flats " on the nut as a second safety and job took around 1 hour,but as said access was the key to making it simpler than it would of been on jacks and stands.
Diff never leaked again from pinion seal.

Have recently drained and resealed the diff plate on new FL 3.2 and that was quite straight forward too,but they really are the only 2 places I could see it leaking from