At this point you're probably going to have to take it to someone and have them weld a nut on top of it to get it out. Often the heating and cooling from welding can also help to loosen the plug and make it easier to get out after.

 

Since nobody suggested it, I assume nobody thinks a case hardened socket coupled with best of breed pneumatic impact wrench would not work.

 

The trouble is that it is an allen not a regular socket so if you put too much torque on that you're gonna either snap the allen in two or round out the hole in the plug.....either is a bad scenario. That is why I usually use a buttefly impact or a 3/8 gun (IR2112) with the power turned way down to prevent that. If it is still stuck I just hit the plug flat on the head with a hammer and then try the impact again. That method has yet to fail me.

 

i had this problem also on my 2000 4runner b/c i did everything you did when i bought it. good thing you did the water pump cuz those leak like hell. im sure they gave you the new metal gasket cuz that is the fix for that leak. i did like most ppl are sayn give it a few taps with small hammer, lots of pb, a stubby allen socket w/ breaker bar.

 

Here's the last resort (after the allen hole has rounded):

 

ditto, 'cept i kept the pipe

 

OK, no biggee. Actually the same as my method above.

 

Sounds like everyone's had their share of this problem.
I TIG welded a 24mm bolt head on and never looked back.
Now I can use the same socket for the manual trans, t-case, and front diff.

 

take a small propane torch and heat around the drain plug for a couple minutes, thus slightly expanding the diff. body while the drain plug stays the same size. not too long though cause the heat will transfer to the drain plug itself. the drain plug should then come out very easy with just the ratchet. heat works wonders on metal.

 

the best method that i ahve ever fond that nto to many peopel know abotu is the heat the bolt quite a bit then touch a wax candle to it the heat draws the wax into the threads and it will some right out. I can honestly say there is no botl i have ever no been able to get out by doing this. Yesterday i took bolts out of an exhuast of and 83 that weere so rusted but they came right otu with heat and wax. Works wonders puts pb blaster to shame.

 

thats what ive done in the past w/ a lot of bolts.