Here's a link for the OM617 rear crankshaft seal thread mentioned by kerry.
Fitting rear crankshaft seal on an OM617
If it is a leaking ball bearing at the end of one of the oil galleries then that probably will be an engine out job too as you need to shove a long rod down the length of the engine to remove them - as my engine is
still out of the car I can't check to see if you could get away with removing the radiator (plus any air conditioning stuff if you have it) and gearbox if you wanted to do that... but to be honest I think it would be easier to "just" remove the engine and gearbox in one go. I can however have a look at the end of the engine in the next few days to see if you can see the ball bearings in the block with the transmission adapter in place - I've got a nasty feeling that needs to come off too - which also means flywheel removal...
I'll check.
So before you commit yourself to either a ball bearing or a rear crankshaft seal replace - read what whunter said!