The current credit industry is a mine field for people who have excellent / good credit histories. In the last few months, lenders have become more stringent in lending to people with adverse credit.
The rate that you have been offered you may feel is high, but due to your history, you are deemed to be classed as a very high risk compared to somone who may have only missed payments. The lender has to make money on the risk that you may not pay them back. Rates are based on risk.
You will be financially associated, if you wern't already, you will both be now due to the joint application. This will have a bearing on your associates credit status too, as you live together, pay the bills etc.
Your wife will have a working relationship with her bank, therefore would always be more likly to be accepted, however, by an outside company, may struggle.
You will struggle to obtain a loan in the current climate with the prime rates that you are looking for. If you are seen to be repaying credit back, it will all contribute towards improving your credit status, in the long run it may be beneficial, but if you don't like the rates, then there's not a lot more that can be said.