The only way of getting your ex-wifes name off the mortgage is to refinance the mortgage into your name.
Your ex-wife also needs to sign a quit claim deed at signing taking her name off of the title of the home. It is actually very simple as long as you qualify for the loan with your income alone. If you do not you are going to need a co-borrower. So it can be difficult in that aspect.
When you are looking for a lender make sure you tell them exactly what you need to do so they can know from the start if you are able to do this loan on your own or if more income is needed to accomplish your refinance.