Don't have a real fix for the problem you now have, but to prevent it I had my dad weld the shock mount bolt in place (as opposed to just using the nut on the back) so that it could never loosen up.
If/when you do this, make sure not to keep using those nuts that screw up the threads, since you can't remove the bolt that easily. /forums/images/graemlins/smile.gif
My suspicion with these setups and their problem is that once the bolt works loose just a hair, every bump is a hammer blow to that mount, and eventually it fails.
GM figured out that was a problem, as they started using bolts that were pressed in like wheel studs.