And the fluid smells like it has been in there for 300K miles. It leaks at the shaft connection, and apparently has been for some time. Is it hard to put in a new seal?

I also cannot find a drain plug???? Silly car, how can I flush it without taking off the plate/gasket? I don't like sucking it out, it will stink up my Mighty Vac! Besides, how much fluid do I put back in? It is not LSD, as far as I know.

Klaus
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Why are Volvos so endearing? Its just a car.

The seal is readily available, or at least it was in Panama 15/20 years ago.
Those differentials are VERY common. Like the guys say, biggest deal is getting the
nut off.
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George Downs, Bartlesville, Heart of the USA!

This may be one of the jobs that you want to sub out. Torque spec, canoodling with the driveshaft and the mess of it might make it a choice for a good rear end shop that has all the stuff handy. It isn't a particularly hard job if you have all the right tools and the patience. The other thing I will add to the list is you might want to think about using synthetic lube in that diff. Especially on the 220s, with the factory 4.56 gears, there is a lot of wear and tear that goes on in the lube.

General Rear Axle thoughts:
If you lift up on the Driveshaft is there movement? There may be an underlining reason for the leak. If that Pinion Nut backed off a bit it would give you movementthat would sometimes result in a small Whine while driving and also cause the pinion to move enough to give you a leak around the seal.
As mentioned, they sell a metal bracket that you screw to the Yoke as a Hold to loosen the Pinion Nut, I have used a Big Ass Cresent wrench to hold the Yoke to loosen and retighten.
If you have done any seal where you have a Shaft popping through it, you can do this one.
Make sure you can get the Oil Fill Off before starting the draining process. Most rear axles, buy 2 quarts of gear oil and make sure you have a way of getting it up and in the Diff.
My weapons of choice:

Google 'SUCTION GUN'
OR
http://www.hopkinsmfg.com/10106.html
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'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me http://home.lyse.net/brox/TonyPage4.html

You drain it by removing the banjo housing cover. You refill it to the level housing plug.

Changing the Pinion seal is pretty easy, But you need a large torque wrench and you need a drilled piece of angle iron to hold the flange while undoing the nut and re torqueing it.