I've done this both ways, once with the bed on, and once with the bed off. And after comparing the two, the amount of effort put into removing the six screws that hold the bed to the frame, and unplugging one plug, it will save you a tremendous amount of cussing than if you didn't take the bed off. So, do your back a favor, take the bed off so your not hunched up inside the fender all day. Also, your arms wont go numb from reaching over your head, and you'll be able to see when you get done, because you wont have rust in your eyes.
As far as how to get the top bolt out, like someone said up top, there is a tab on the nutthat keeps it from spinning, if that tab breaks, no big deal you can fit a wrench on it, if you have the bed off. If the tab breaks and you leave the bed on, good luck.

Then, once the nut is off, just use a pnuematic impact and let it spin in reverse, eventually it will thread itself out.
OOPS, didn't read your post below this one, most of the time guys just reply in their original thread to say thanks, not with a completely new post. Oh well, my stupidity for not reading ahead.