Audrina Patridge and her sister Casey had a few drinks in West Hollywood the night before last -- and the evening played out like a lame-ass episode of "The Hills."

TMZ spies overheard Audrina say she's now totally exclusive with borefriend/slacker Justin Bobby -- and LC is giving her the same cold shoulder she gave Heidi after the whole Spencer debacle.

And get this: Audrina said she might move out of Lauren's pad and shack up with Justin Bobby! Lucky bastard.