Do not sync your iPhone with iTunes or your iPhone's backup will be updated to reflect the deleted SMS conversation.
Without syncing or updating your iPhone's backup, restore your iPhone with iTunes from your iPhone's backup. When selecting Restore under the Summary tab for your iPhone sync preferences, if you are prompted to update your iPhone's backup, decline.
This will completely erase your iPhone followed by reinstalling the firmware. After the firmware is reinstalled, you will be prompted to restore your iPhone from your iPhone's backup, which is the default selection. This must be followed by a sync to retransfer your selected iTunes content and any photos from your computer to your iPhone.