To the Internet! I Googled a bit and eventually turned up saukvalley.com, "serving Dixon, Sterling & Rock Falls" and a bunch of other piddly little places no one cares about. And by poking around through the local news archives, I found an article about the bombing in Morrison, which contained both the name and the address of the owner of the destroyed mailbox. I keyed the address into Yahoo! Maps and saw with some glee that it was less than a mile off US 30 -- the very road I would be taking to get to Cedar Rapids.
I'm so stopping there.