It does sound like the rear seal. I would not wait for the clutch to fail, why make this more expensive? You will have to remove the transmission, bellhousing and oil pan I suspect. I am not familiar with this engine in particular but, it sounds like you have a leak large enough that it needs to be addressed. There are a couple of more technical forums for the G-wagen that would be of great help. They compete with this forum so mail me off the board and I will be happy to direct you.
brenth@wcta.net