You think hanging off the bolt is bad? I did my first diff. oil change just a month or so into my endeavors as a home mechanic on my 300cdt, and I got the drain plug out fine, but couldn't budge the fill plug. Wrong order to do it in, I know. I had no guidance except for the Haynes book, and no experience with cars whatsoever. Here's the good part:

To break the bolt loose (after 3 unfruitful hours of hammering, wrenching, and liquid wrenching), I cinched down two US military equipment straps locked together, one end to the handle of the allen wrench and the other to the towing hitch mounted on the back of my dad's Ford. I drove the Ford forwards a foot or two, praying hugely, and lo and behold! the plug had loosened like nothing!! And it didn't affect my diff. whatsoever. Beginner's luck, but hey, it worked.