Do you all drug test? If so, is there a way you can initiate one for her, even if it means calling HR anonymously and suggesting it?
Sounds like time to keep track of the cash accountability on the shifts that she, or her little buddy, works. If you make a paper trail for it, you'll probably be amazed at how quickly the evidence will stack itself against her, if that's what's really going on.
As far as the insuboordination thing goes, have you talked to her about it? Maybe she's not aware that you know of her actions (as far as talking about you behind your back and such) and if you confront her she may be scared into stopping. If not, and your boss still won't do anything about it, then I would go to the next level beyond him. Not to get him in trouble, but because things like that will begin to affect the way customers view your store, if it hasn't already, and they can really hurt business. If you put it that way, I bet the next level of command will be glad you went to them about it.
I worked in a supervisory role handling the cash office for years at a store... and I am so glad I got out instead of going into management, for similar reasons. Politics can be hell.
Good luck!