Note to Garageband Users: Export Song to Disk ADDS DISTORTION
Well, I have found a workaround: Instead of Share/Export Song to Disk, which creates the AAC/m4a file directly from the Garageband project, use Share/Send Song to iTunes. The song will then arrive in iTunes as an uncompressed AIFF file, which sounds just as perfect as the playback from within the Garageband project. At that point - in iTunes - go to Advanced/Create AAC Version and the song will be duplicated from the AIFF version to an AAC/m4a file. No more distortion! You can then drag and drop the AAC file from iTunes to your desktop, which effectively duplicates it, as the original stays in iTunes. Yeah, it's quite a few steps, but the AAC conversions Garageband does are so bad as to be unusable.
For some reason, Garageband does not convert from a project directly to AAC/m4a worth a darn, but iTunes does just fine converting from AIFF to AAC. No, I have no idea why, and yes, it is very un-Apple like.
Since this has been driving me crazy FOR SEVERAL MONTHS, I thought I'd put my solution on the intertubes for anyone else who is having this problem. I had actually gone back to Protools LE and even set up Cubase LE because of this problem, so it's nice to be back on Easy Street with Garageband: If you just record solo stereo guitar, you don't need the complexity of Protools or Cubase, not to mention the hardware limitations of Protools.
Now I can get back to my recording project!