D4D final leak

"67, D4D, 78A7177, direct drive. Guess it was bound to happen, It has been just sitting there since spring but for a couple hours of use two weeks ago. There is now a small but growing spot of oil on the inner track below the left hand rear drive sprocket. The shop manual I have does not indicate what is involved in replacing a final drive outer seal(s). How big a deal is this?

D4D final leak