1980 300SD Differential Sleeve Leaking

On my 1980 300SD one of the rubber sleeves on the rear axle shaft where it enters the differential is starting to weep a small amount of oil.
Some questions:
How risky is it to just monitor this for a while and only deal with it if the leak becomes more substantial? My main concern here would be that water getting inside could also be a problem and then result in differential failure. Have people seen this happen? And I guess the other risk would be that the leak could suddenly become real bad, resulting in total diff oil loss and failure, but it seems this is fairly unlikely.
I do most repair work myself, am fairly well skilled, have a good set of shop tools, etc. But looking at the service manual, it looks like this is one of the jobs better left to a pro. Has anyone done it and opinions?
The local repair shop that I use has quoted $103 for parts and 7 hours labor to do the job. The car is otherwise in excellent shape, with only 123K on her.
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