I have a 1984 Ford F-150 4x4 and I am having trouble removing the rear differential fill plug. The fill plug is really rusty and there is almost nothing left of the square hole in the center I tried turning it out and the 3/8 drive extention wants to slip in the hole. What would be the best way to try to remove the plug?

 

Get a 3/8 sq. bar with sharp edges, bend it 90 deg with heat, make sure it is in all the way, then break it free. Replace with new plug.
Note, some people use a tread locker on the threads.
If above will not work you might be able to use an easyout.

 

If all else fails, buy an aftermarket aluminum cover with fill and drain holes on the cover. As an added bonus, you also get carrier side bearing support screws.

 

Mine also has a bit more capacity too from what I recall. Nothing huge but it's around 1/2 quart I think...

 

I'm thinking about putting a bolt in the hole of the plug and welding it to the plug. Then trying to turn the plug out with the bolt. The heat from welding it should also help break the plug loose. I know I have to watch how hot I get the differential housing because of the seals and the fluid in it, but I think this will work. What do you think?

 

I like StrangeRanger's idea better, but your idea could work.

 

Any other suggestions for getting the fill plug out?

 

I would put a new aftermarket cover on there. Not to hard to find a Ford 8.8 diff cover.

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