First, a shout out to Chris Adamson over at O'Reilly for the original inspiration for this. His article and comments led me down the right path.
Gigavox's Levelator don't play nice with Garageband as GB outputs compressed files and the GVL needs the uncompressed files to work. How cool would it be to have your final output from Garageband levelated (is that a term)?
1) Delete the “Podcast” track from Garageband. You won't get the chapter marks and all the fancy enhanced Podcast stuff (but you can re-import your levelated uncompressed file into GB if you need that stuff).
2) Under the “Share” menu, select “Send Song to iTunes.” This will send a mixed, uncompressed AIFF file to iTunes.
3) Levelate that file.
4) Import the Levelated file into iTunes and convert to MP3.