Gotta go with lay the fence down. If it's 4 strands of barbed wire, might even be smarter to cut it and then splice it back together, assuming you have access to stretchers. A come along makes a good stretcher, BTW. Odds are, if that fence has been there a while, it's already stretched some, and isn't nearly as taught as it was the day it went up. Just be careful of tensile strength of the wire. If you've never heard that snap from about 50 yards away, and the whistling of the wire coming toward you, count yourself lucky. There's nothing a lot more inconvenient than a huge ball of barbed wire coming at you.
Looks like the post might be compromised if you try to cut it before pulling the base to one side. Once I connected to it with the tractor and got it started moving, I'd keep dragging as long as I could and let the whole thing come down, then get rid of the waste. That's a dangerous snag, and very appropriately called a widow maker. Ther looks to be enough rot on the bottom end that it will start moving fairly easy, and the majority of the tree while rotted, it isn't decayed like the stump end. Just make sure to tie as low as possible to the log, and same on the tractor end. If you can move it enough to clear the post, then start taking chunks off (undercut first, of course). I don't think I'd try to completely cut it out, though. The top will start coming apart and falling. With it on that sharp an angle, you have a lot less to worry about not being under the top. Odds are, while the top may be rotted enough to break, it's still plenty solid to break bones when it comes down from that height.
I (barely) remember going out coon hunting one night with my stepdad and a neighbor, Mr. Jim Stout. Dogs treed, we went to them, and the coon looked down ONCE. Bright moonlit night, so he'd only look up. Grabbed a vine to make some noise and try to get him to look down, without noticing first that the tree was very dead. I did notice, however, that it was VERY tall. Next thing I know, my stepdad and Mr. Stout were picking me up. A 6-inch limb had broken off and came down hitting me right at the base of the neck and knocked me out cold as a wedge. I'd been taught to not look up if I heard the crack of something breaking, and that's probably what saved my life. I still have a healthy fear of dead snags.
