How do you remove the plug from the differential. I want to check the fluid in it.

I took one of my 3/8 socket with a breaker and twisted the end off the extension. I guess the weak point is where the locking detent is in the extension because it split there.

Is there a special tool for this?

 

Nope, 3/8 extension like you used. Usually the problem is you screw up the plug if you don't clean it out first and get a shallow bite. You're the first I've ever heard of that had the extension break. Take a torch and heat around the plug for a minute or two and try it again.

 

I used an extension made for an impact wrench and heated it a couple of times. It finally came loose.