In Windows Explorer, try browsing to the folder below, open the 'graphicoptions.xml' file in Notepad (or other text editor) and make sure the proper resolution is listed. Maybe a widescreen resolution entered here will work even if it doesn't show up in the game options.
XP
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Profile
Vista/7
C:\Users\ %username% \AppData\Local\Divinity 2\Profile
Try running the game as administrator (and in Vista/7 disable desktop composition in the compatibility options of the program/shortcut). With D2:ED someone mentioned higher game resolutions becoming available by running as administrator, so maybe that will help with additional widescreen options.
