Thanks sparrowlight! I'll try it tonight after I get home.
I assumed it was still jailbroken because (i) I didn't uninstall the jailbreak and (ii) either Geekmaster or ixtab (can't recall which) posted times that if it was jailbroken before the 5.1 update, it was still jailbroken. It seemed to work, since it gave me the "update kindle" option in the settings submenu (it was not greyed out), and it began the process and restarted. Then I got the error message. But if this doesn't work, I'll try uniinstalling and then reinstalling. Thanks again.