Export from garageband to audacity
GarageBand is basically a simple version Logic, in a similar way that iPhoto is a simple version of Aperture.

What you want to do is easily done in Logic, and it also has the functionality within it that you probably wouldn't even need to export it to Audacity at all.

You can import anything created in GarageBand directly into Logic.

(if it's something you can't justify buying Logic to do, you could get someone with Logic to export the GarageBand tracks for you)