When you reach up in the body while you are spinning the bolt from the outside what happens? If the outside is spinning then you aren't grabbing it right with the vice grips. If it is rusted together it is not going to spin.
I had the same problem. I had to go buy smaller vice grips so I could really get them in there and you can feel the part of the bolt they need to go on just get the vice grips on it so that when it turns it is going to do about a 1/4 turn then the vice grips will not be able to turn any more and voila`.
The really fun one is the front top one closest to the cab, It's way up there (hopefully those are good to you). Funniest thing when I was trying to do it with the big vice grips I just couldn't get sufficient pressure with one hand to lock the vice grips tight enough so I squeezed my other hand in and got it locked but my arms were stuck
I panicked for like half a second then realized I got them in I can get them out and freaking out wasn't going to help.
Hopefully this helps what I thought was going to be an easy 1 1/2 hour job turned into a 5 hour job.