Well, I had two of my keeper nuts do the same thing, and got a bit barbaric in my solution to the problem.

I ended up cutting off the top of the bolt below the guide rod mount bracket, allowing me to remove the whole assembly.
Then I sliced the bolt off as close to the chassis as I could, before pushing it up into the hole it's hanging down through.
The barbaric item is that I had to open the top of the little opening that holds the end of the outer tab in place, to let the old bolt come out. I used a small reciprocating saw with a metal cutting blade to cut an arch in the top of that opening, rather than it being rectangular as it started. With a little fiddling and a pair of needle nose pliers, I was able to fish the stub of the bolt and the nut out of the suspension cavity.
(A note here: I touched up the cut edges with rust preventing paint to hopefully keep any rust from starting in the area that I disturbed during this mess.)

This was a royal pain, and I probably wouldn't have done it this way if I hadn't had a few spare parts on hand from my parts car.

Knowing what I know now, I might have just attempted to change the guide rod mount in place on the car, though I'm not sure of the clearances behind it while it's installed.

Once it was all done and told, it made an amazing difference in the ride of the car, so well worth the trouble in my book.

__________________
-Josh
Testing the cheap Mercedes axiom, one bolt at a time...