Please go to WPML > String Translation and use the text search field to search for "Ship to a different address". You can use the additional checkboxes to search for the exact text and to include translations.
If you can not find the string, then you need to register it first on String Translation.
I suggest you go to WPML > String Translation and enable the option to "Render strings while loading the page" and then refresh the page on which the string is displayed. After that disable the option.