Take a small screwdriver and clean the hole out before starting. Tap a ⅜ extension in there with a hammer and you'll probably have good luck. If it doesn't want to loosen try to tighten it a bit just to crack it loose. I usually have good luck with those. Unless of course the last guy in there was in a hurry and cross threaded it. (thinking "not my problem, the next guy can deal with it")

 

The 3/8" extension I tried fits pretty loosely... When I start to turn I can feel it start to want to lift out. If I don't put pressure on it, it will turn right out. I think someone else tried and went, oh well, the next guy will have to worry about it. I shot it with some PB Power Blast and I'm hoping that will help.

The funny part is, I went to the local parts store quite a few times on this project. The last time I was in there I said, "Who wants to bet I have to come back for the fill plug." I never even looked for it before I said that...

 

Definitely!

Have a look at this:
Rear Axle Service

 

Very nice! I'm pretty familiar with that view! I just wish my fill plug was that square. I'll try to get a pic of mine next time I'm under there. It looks more like a hex head.

 

If its too stripped, sometimes theres just enough edge to grab with vice grips.

 

That's what I usually do but this thing is flush just like the link that swartlkk gave. I'm afraid I might have to heat it a bit...

 

know anybody with a star-bit set?

 

I think I could probably find one. One of my co-workers has a set to remove damaged bolts, he just has to find it...

 

I just had the same problem with the sliders. Three out of four were in so bad that I had to cut the heads off with a hacksaw to salvage the rubber boot. Well worth the money for two new brackets, 4 pins and a lot of jelly!

 

I got my new brackets and bolts while I was on vacation. Just got them installed and they worked wonderfully! I'm so glad that's done!

All I have left is the stupid differential plug to remove.