I wanted to pull my shield today to clean out debris but the second bolt from the rear on the passenger side just spins. Wondering what is on the other side hold it. A nut welded to frame? Or just a captive nut that may have corroded?

 

On my 2007 V6 4.0 I think all the nuts are welded to the frame, but also think the skid plate is part of the fuel tank because we had to put a transmission jack under the tank during our transplant project and felt the tank was going to come down on us. Some previous worker had neglected to put 3 of the 6 bolts back in so we had to support the tank to get them in...

 

correct, it holds that tank in place. You drop the skid, you drop the tank!