Is there anyway to back up the text in iMessages?

The backups are in your user's ~/Library/Application Support/MobileSync/Backup folder, although they are not in a format that is readily usable (see Locate backups of your iPhone, iPad, and iPod touch - Apple Support). I would make a copy of the latest and stash it somewhere so that you always have it.

You can use a utility application such as iBackup Viewer to view and/or extract items from a regular iTunes backup of your iPhone. There are a few of these kinds of utilities that vary in what they are capable of doing and the amount of money they want for doing it (most have limited 'lite' or free versions), so check around for one that has the features you want.

The messages would be saved in a regular backup.