There is a relatively fragile cage nut sitting inside the hole there that has, by your description, broken away from the stamped body it's held in by (which is what happens when you hit it with an impact and it's rusted in place) Basically, your bushing support/washer rusted through (they're hardly prepped for corrosion protection). Your situation as it stands looks GOOD. Hardly any body rust to speak of.
Your plan of action should involve sourcing a new bolt, washer, and bushings. You can get the bolt and washer from GM, and if you'd like to upgrade your bushings, you can use the Energy Suspension polyurethane frame bushings designed for the GMT325 (S10, Blazer) platform while you're there. If you're keeping the car for a while, and not doing a patch job, I'd HIGHLY recommend sourcing 6 new support washers to go with those new bushings, because I'd bet they're all pretty rusty.
As far as removing what's there...it's not exactly EASY to get that cage nut out of the way for a new nut and washer to go in as it's still going to be held in by the mangled metal that's letting it spin freely.
My attempt of a solution would be to whack upward on that bolt until the nut breaks out of the cage, then you can cut the bolt in half with a cutting wheel/grinder and pull the remaining bolt and nut out through the access hole.
The other thing is, if the inner rear bolts start spinning (the ones without the access hole), you'll need to cut an access hole in the body to get the bits out, and then re-install from there.
I did this whole operation on my 96 PAU, and I have some pictures for guidance if I can be of any help. But I wouldn't worry, considering how bad it could be, you're in good shape there.
I was going to look for my pictures, but the pictures of Peter's 92 SSEi unibody repair shows a lot more angles of exactly what you're looking at.
http://s15.photobucket.com/user/peterg2 ... t=3&page=1
If I can find mine, I'll post those too, since it shows the un-rusted cage nut you're probably dealing with.
