Hooks,
Let me first just say that my most heartfelt sympathies go out to you and your family for the difficulties you are experiencing. That being said, I feel that I need to provide what insight I may have, having raised my 21 year old daughter without Mother.
Girls are different the boys insomuch that they react differently to situations when presented. I am sure that you still think of her as your little girl at times and am also quite certain that you are questioning where you "went wrong". I assure you this does not apply. If you are certain that she is on drugs and not just pot or beers but heavier than you must take action. I feel that this is not only your duty as a father but also your best course of action. I do not say that to be harsh but help explain the roles you may take.
A seventeen year old is a baby in the big picture of life and she is still extremely vulnerable. I would sit her down and simply tell her of your predicament in the most calm and adult manner you can. Explain to her that you are struggling with what to do, not because you want to punish her or run her life but that you love her dearly and always have and everytime she leaves or you pick her up from the police that a piece of you suffers twice as much because you don't know how to make things better for her. Tell her even though you are her father that you made mistakes too when you were young. Ask her if she is using drugs and why, what does it do for her? In other words start to estabish a relationship of trust. Reassure her that you will not stand idly by and watch her piss her life away to a life of probaton or jail. Be honest and personable and open and ask her what your solution should be. Inform her that you are willing to do whatever it takes to re-establish yourself as the father figure that she so desires and if that means rehab then so be it and then tell her you will go with her if need be. Let her know that you miss her mother also but that you cannot stand to lose her too. Tell her about the dread that is tearing you apart and the struglles you are having on your own. Confide in her, assure her that you want to take a larger role in her life if she will let you. Remember Hooks, this is the same girl that couldn't get enough of the swing at the playground or whatever when little, only now she is reaching out in the only way she knows how. I am confident you will respond the right way.
I do not intend this to be harsh or demeaning of your relationship only to give you my advice as I had to do the same some time ago. Remember she is insecure, depressed and sad and is reacting how many would considering the pressure she is under. She needs guidance, and you are just the guy to give it to her, again, try to establish the best relationship you can. Good luck