How can I remove my wife's name from my auto loan?

I don't think you can.  Your wife owes an obligation to the lender.  The lender is under no obligation to release her from her obligation.  Of course it will if you meet its criteria for refinancing, but it seems that you do not.  The best you can do is to contract with your wife that, if she is ever called upon to pay anything on the loan, you will indemnify her.  Of course, if she ever had to pay on the loan, it probably means that you were unable to pay it, and thus would be unable to indemnify her anyway, but it's better than nothing.  If you pay the loan on time, however, your wife has nothing to worry about.