It seems my 1977 300D has developed a leak at the rear axle. I put the car away for winter and opened it up this afternoon and the cardboard under the rear end is soaked. I circled the leak in the photo below. I'm assuming it's a seal that needs replaced. Anyone know what seal I need?

 

I would really prefer to replace the actual seal rather than use silicon. My car is mint, with only 116,000mi and it has a lot of sedimentary value. I don't want to start skimping on up keep.

I have been looking at Classic Car Parts Giant, but can't find anything that is simply labeled as a differential seal. There is a Diff. Side Cover Seal, anyone know if that is the correct part? Other than that all they have are assorted gasket and seal kits.

I have a local shop that does all my work, but he isn't a specialist on diesels or foreign autos. So I would prefer to get the part for him and just have him install it.