The first time I tried to remove this drain plug (about 1993), I could not remove it, so I took it to a friendly repair shop, that loosened it for me, by using an impact wrench.

Well, I must have really cranked down on it last time I changed my rear axle oil, because I was not able to remove it. After really tearing up the bolt head, I used a grinder to cut a slit in the head, so I could use an impact screwdriver to try to loosen it. No go with that method.

So, I purchased a set of 25 Irwin Bolt Extractors, and they worked perfectly!

http://www.amazon.com/Irwin-Industrial-53227-Multi-Spline-Extractor/dp/B0002SRG66/ref=sr_1_3?ie=UTF8&qid=1391934074&sr=8-3&keywords=irwin+bolt+extractor

(I actually purchased a set off of ebay, for $52, free shipping and no sales tax)

I think this axle bolt is made of a metal that is somewhere between silly putty and lead, as far as how soft it is.

I was able to easily drill into the metal, then, guide the extractor into place. I used my 2 foot, 1/2 inch breaker bar to apply force, then, the tension on the bold snapped, and I was able to easily unscrew the mangled plug.

I may be a little fanatical about changing the rear axle oil, as I change it every 50,000 miles, but I feel its cheap preventive maintenance. This time, I used a new genuine Volvo magnetic drain plug, new copper seal ring, and a little Permatex anti-seize, and torqued the plug to 20 ft lbs.
--
If it needs to be maintained, repaired or replaced on a 1990 240, I've probably done it. '90 240DL, 350,913 miles, will she make it to 400K ?? >>You haven't really worked on a car until you draw blood<< :-}