That's called a "captive nut". The nut itself is loose but, its captured and held in place by a sheet-metal tab. This allows some movement of the nut to get things aligned on the assembly line, and the tab keeps it from spinning (not requiring a wrench on the backside). This a very common problem on old muscle car body mounts...the bolt breaks, (or the tab breaks and allows the nut to spin). Either way, it sucks. You got a few options:
ONE:
Drill it from the bottom up and use an extractor...(like Renegade says: you'll need space to do this). Extractors are a problem within themselves, and I only have a 30% success rate with them. They only work when you are trying to back a bolt out. I'm assuming that your bolt snapped while you were loosening it? If so...a standard RH extractor will probably not work. This is because the threads galled and seized while loosening. So you might want to remove the stub by tightening it in the other direction. I don't know if such a critter exists...but what you need is a "left-hand extractor". This will work just like a normal one but, it will spin the bolt in the opposite direction, up and out thru the top of the captive nut. Pluck the stub out with a magnet...problem solved.
TWO:
Stick a plasma or torch tip into the bolt hole and melt the entire captive nut & broken bolt out of the way. Replace the nut with a new one via the tiny round window, but you'll have to find a way to keep it from spinning. Quick, easy, and ugly!
THREE:
Cut an access hole in the frame, and remove the nut & broken bolt from the side. If you do this you will need to make sure you either leave a big enough hole to get a wrench in there (to hold the new nut)...or you will have to mimic the captive nut idea by making your own new captive nut. The H3 frame is very thin at this point and you can easily cut a window in it with a cut-off wheel.
I do have a fourth idea. Since the bolt is broke right where it enters the nut...it might be easier to continue threading it upward (tightening) and remove it from the top. This will require you to somehow spin the remaining piece up and out of the nut. You'll probably still need a bigger window in the frame. Normally vise grips would be used but, in tight spots like this...I have had great success threading a nut onto the remaining bolt stub, and spot welding it to the stub. This essentially makes it into a "bolt" and you simply thread it up and out with a wrench. This would leave the factory captive nut in place, and all you would have to do is get a new axle mounting bolt, and patch the hole you cut in the frame.
Whatever method...Good luck. I've been there...its a bitch...no easy fix.
