How did you fill the diff from the drain hole, did you turn the truck upside down?
Here's how I get the tough ones off: soak with PB blaster for several days. Use a good, 6-point 24mm socket, not a 12-point. and 1/2" drive. I've got one you can borrow if needed. File that bolt like Wes suggested until that socket will swallow the whole bolt head and rest on the diff cover. Put breaker bar on more or less parallel with the axle, aimed down a little, handle towards DS tire. Then use a small bottle jack (or similar) to push up on the handle from below. Use all your weight to press on the backside of the socket keeping it on the bolt, while you increase the upward (lefty looosy) pressure with the jack. the jack does the heavy work, you keep the socket in place.
Once you get it out upgrade to a new one, like a Corrola one with the 14mm hex driver or at a minumum, a new bolt & new crush washer.
82 fj40, 99 uzj100, 06 gx470
97 DR650, 05 525EXC, 01 XT225, 89 TW200, 09 TTR125, 07 TTR90, 07 CRF50