The last long stretch of unimproved road resulted in an intermittent rattle that I didn't manage to track down until this Sunday.
The drivers side rear shock was presumably installed without a washer on top of the top rubber grommet. When I found it, the top grommet was gone, but the nut was still in place. This left the shock loose if/when the body moved upward faster than the shock could.
There are scrapes all over the little shield that protects the fuel tank. Good thing that that's there.
After sourcing another grommet and washer (and making them fit the thicker shaft of my shocks), it was a pretty simple repair. Tightening the nut down did make me wish for some ratcheting wrenches though.
So, if you think your shocks might not be working that well, check to make sure they are in fact attached to the vehicle. In this case it was pretty hard to notice, since it would slip right back into place anytime the van was at rest.