splitting a stereo track into two tracks

Thanks again, Hangtime. Your method will get the sound right, but technically it is not creating any mono tracks, just deleting one side of the stereo signal per channel. To get a regular single track I found this to work:

This only requires Garageband and is quick.

I assume your track has a single region: if not you will have to shift-click to select all the regions in the track that you want to convert.

1. Click on the track that you want to convert. The single region should automatically be highlighted. Copy the contents.
2. Command-D or >Track>New Track with duplicate settings to create a new track
3. Command-V to paste the original contents to the new track
4. Shift-Click on the original region(s) to highlight it again Now two tracks are selected.
5. Command-J to join the tracks, and select "Mono" for the mix-down.
6. The mix-down will replace the original stereo track - you can now click on the temporary track and Delete it.

The key here is the little window after the cmd-J asking if you want the resultant track stereo or mono.