I have a Dana 50 in m 95 F250, and I have found a Dana 60 to replace it.
The guy is trying to sell it for $950.
I asked if anything was wrong with it, he said the things needed replace..
Backing plates
Pinion seals

Where can I find these two things, and is it enough to get the axle cheaper to replace these two?

 

If the backing plates are missing just leave them off no big deal if damaged take them off. If you want them it will be a junkyard or Ford. Pinion seal's most auto parts houses should have them. easy stuff