I was just told that my 2008 XC70 with 81,000 miles has "final drive failure".

I believe you are referring to the rear differential (carrier). I unfortunately live in the center of the US away from the specialists that have repaired these issues before. Lambs on the east coast specializes in rebuilding these. I took my chances and purchased a used unit put new seals on it transfered my haldex module and put it in myself in about 10 hours. It is substantially faster when in a shop (4hrs) but doing it myself cost me less than $400 in total. I believe shops can do it between $800 and $2000 depending on if it is used or rebuilt. Once you replace it have the fluids changed every 50k miles at most. I drove mine for about 60k miles before actually replacing it. It is very loud and unpleasant but Generally they do not lock up as long as they have fluid in them.