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