I wouldn't say that most are AU's, but a lot of them are. I tend to like those the best. To me, they are original stories with the J's as the lead actors in them, sort of like they are playing the part of themselves.

I loved that Anne of Green Gables one - so delightful! There are some J2 AU recs located here to give you a taste of what's out there: http://sites.google.com/site/j2authemedficrecs/ Some of these also have podfics.

The best thing I can recommend is just to cruise around and try some different J2 podfics - you'll discover some favorite readers pretty quickly, and then you can see what else they've recorded.

Hope this helps!