You would need to use a tool that can examine the backup on your computer. I would try the decipher text message program. There is a free trial (which I've used before) and it lets you poke around and see what iMessages are in your backup. From there you can export them.
do you have any older back ups saved, think that would be easier if so, i have a program called phone view that saves texts + more from all my phones in case...not sure if that would help you now but check it out