I have a CJ7 with an AMC 20 rear axle. I finally got the cover off to fix a leak. Now, the fill plug is so rusted that the allen wrench turned in the socket and the plug is still frozen. Do I drill the thing out, or what? I have gone through a bunch of rust buster stuff and bought the right tools to do the job...nothing. I have to refill the diff. Somebody tell me the best thing to do.    

Concentrated heat from a MAPP torch played directly on the plug. Don't warp the cover with too much heat transmitting to the cover.

Let it cool and then use a manual impact wrench, one you hit with a hammer, not air.

If the plug is already rounded out, probably a new cover.

Not to rub salt into the wound, but for anybody else reading this, always remove the filler plug first before draining fluid out of anything.  

I've used consentrated heat then cold water to shrink it, also you can weld a nut to the plug and use a socket instead of the allen...  

I welded a nut on a friends cover before for the same reason. Just be careful not to weld the plug to the cover. I migged it with a couple of good tacks. With the heat from the weld and having a nut to put a box end wrench on, it came right off.  

yep they do, i usually just fill in the center of the nut with the 110 and innershield wire.