The slip in part of my tie downs has a bit of slack in them. So I would get a large adjustable wrench and see if a little left and then right, then left, and so forth might work the removable part loose. If it is just rust, it may break the bond loose enough to be able to squirt some lube up in there and get it to slide out. If that doesn't work, or a few good taps with a large hammer, I would take the entire part off as it would be much easier and safer to use a torch on it out from under the truck. Twisting and heating it should work. Other possibility would be to just leave it in place and use it. I keep my tie downs installed all the time, even when I an pulling our 5th wheel as they don't get in the way of anything. However they sure hurt to walk into them though. LOL