I have an old John Deere 726. I replaced the engine on it last year (pics here: https://drive.google.com/file/d/0B7J...ew?usp=sharing) and had some difficulty getting a belt the right size for the auger pulleys. Finally found one and it worked great but appears to have stretched a bit and now the belt is slipping when I hit snow over an inch deep. I didn't realize initially there was even an adjustment for this belt and was stoked to find it, but I cannot for the life of me get this thing to loosen.

I put it in a tub of carb cleaner overnight to try to break any grime in the threads then today bought some PB Blaster but haven't had luck with that either. I'm using a big plumbing wrench and feel like the bar is going to snap before this thread comes loose.

You can see a picture of the piece I'm working on here: https://drive.google.com/file/d/0B7J...ew?usp=sharing

I believe it is ball joint AM33457 and rod M83161.

Any ideas? I'm starting to think I may just need to replace it. Right now I have some Romex wire strung around it as a workaround but that is a temporary hack. I also considered cutting it down and adding a turnbuckle in the middle but wan't sure how to affix it to the bar (I don't know how to weld).

Welcome to the forum, I have to say right away I don't have experience with your JD model but I can help to get your piece loosened.
You need a torch (small propane one can work but better if bigger)
and a bottle of cold water, heat the piece good and once hot close the torch and quickly pour the water on the piece, then you can start with your pipe wrench and see if it budges even very slight and if so just wrench it both ways and you will see that it will move more and more. Good Luck and put some anti-seize after.

YEAH what NORM said. ALOHA from the frozen TUNDRA.

Thanks for the help! Apparently there was nothing that was going to loosen this connection. After torching then applying water and cranking on it I was still not able to loosen it so I tried a few more times, cranking harder each time until the last time the rod just broke. Not sure if someone put loctite in the threads at one point or what, but they are not budging. Now of to find a replacement part.

This is sad as I have never failed using that method, however was the rod much longer inside the threaded sleeve and beyond?

I'm not sure how far the rod goes into the sleeve because I was not able to unscrew it, but it is a fairly long rod with a 90 bend on the opposite end. In order to keep it from slipping I put the 90 bend in my vise so it had a long length of rod that was twisting when I was turning so it was my bad for torquing that way. I just couldn't get a grip on the rod otherwise.