If her card is over-limit (presumably getting fees because of that), would it not finicially be better to pay hers down first (at least to under the limit)?
The reason she is mad, is because you seem to be spending money where it does not directly help. "Her" card that is currently over-limit, if brought down below the limit, could then be used again (even if that is finicially irresponsible). Quite simply, she feels you are placing the credit card company above your own well-being.
I would explain, calmly, that it is disturbing that she "flies into complete rages" about things that are best discussed and considered calmly, that you don't know why it bothered her, and that if she expects you to understand and be able to respond to whatever her feelings on the subject may be, then she needs to tell you what it is that she feels. (Use "I statements"... "It disburbs me when..." "I don't undestand why you...")
Assuming you survive, then, later, you can discuss the finicial merits of paying off the CC (i.e. it could easily save you $75 per month when you refiniance), and why you feel the money is better spent on the "bad" debt as opposed to elsewhere.