Almost the same thing happened to mine. I went to tighten the last bolt down, and it just spun and got no tighter.
We had to cut a small hole in the chassis to expose the bolt. I didn't get a good look at it because I wasn't really the one that did it but I think the rear bolts on the subframe are pinched with something similar to sheet metal. I thought it was a nut that broke free from a weld, but no. So we just moved the metal back in place to hold the end of the bolt, and tightened down. Worked perfectly.
Just covered the hole with a piece of metal, welded it shut, and sprayed a little black paint on it. Can never tell it was cut.
Someone will probably post an easier way but that was all I could think to do.
