Hm, I don't see a runaway in this case. No money, leaving all her belongings (her dog, shoes and car) behind, not informing her dad (to whom she was close), her last intention clearly was to go to her boyfriend, and all of a sudden she says at 5 am in the morning: "No, wait, I'm hitchhiking to Mexico!"
If she vanished with her car after breaking up with her bf, yeah, maybe. But as things stand, it would be much more likely that she broke down and committed suicide. However, IMO, the most likely scenario is still that someone grabbed her. If we wouldn't have the calls to the cab, it would be a textbook example for an abduction: walks out the door to go somewhere, is never seen again, her car later found abandoned in another place.
If she really left voluntarily somehow, I swear to God I'm gonna abduct her myself!
But don't worry, I will make sure to leave some more clues for you guys.