This is one of my favorite fics -it is an oldie but a goodie: Res Gestae by dreamlittleyo. It involves the brothers being cursed and they go on a road trip together to get it fixed. NOTE: It's not clear to me from your description, so I will advise you that this is NOT "always a girl Dean", and it is wincest.

http://archiveofourown.org/works/178612/chapters/262769

Edited 2016-07-11 01:23 am (UTC)

My favorite male!Sam/female!Dean fic is fayjay's "Not Time's Fool" (http://archiveofourown.org/works/4720): "A story in 21 parts, featuring an Ancient Greek curse, an unexpected metamorphosis, adventures in pool sharking, numerous shots of tequila, a nun outfit, zombies, angels, demons, kidnappings, startling discoveries about old acquaintances, massage, a game of strip poker, girl-on-girl action, girl-on-boy action, and boy-on-boy action. Despite my expectations or inclinations, it swiftly became clear that this story was going to be Sam/Dean. And, indeed, it is. Because my muse likes to fuck with me. It also features Girl!Dean - but don't let that put you off. It's still Dean, bless him, despite the sudden girlsuit." This is also Wincest and notalwaysagirl!Dean.

You're the One That I Still Miss (http://archiveofourown.org/works/508886) by tebtosca (genderswap, wincest): A curse sends Dean running to Stanford to hide out, but an unexpected life with Sam keeps him there.