If it is the pinion seal then DO make sure you get someone who knows what they are doing (preferably an experienced indie). It's very easy to wreck the pinion bearings and then you are looking at a diff rebuild.
Just had mine done on my R129. Mercedes dealer quoted me over £800 so I took it to AMC in Earlsfied who did it for £380. Parts cos about a tenner. It's all labour. Seeing as the cost of parts is negligible, it seems worthwhile getting all three seals done whilst they've got it all apart, even if they are not all leaking.