Try right clicking the executable and run the game as administrator (..\SteamApps\common\Divinity - Original Sin\Shipping\EoCApp.exe). That helped someone in Divinity 2 that had missing resolutions (though in that case it was only higher resolutions that were missing).
Did you try changing the display mode to fake fullscreen or windowed?
You can also try deleting/renaming the graphicSettings.lsx file (..\users\%username%\My Documents\Larian Studios\Divinity Original Sin), and see if getting the game to recreate it will fix the resolution options.
Try changing your desktop resolution, and see if that gets any resolutions added to the dropdown list in the game settings (though the game will not list anything higher than your current desktop resolution).
You can edit the graphicSettings.lsx in Wordpad, or other text editor, to manually change the resolution. Search for the section:
</node>
<node id="ConfigEntry">
<attribute id="MapKey" value="
ScreenHeight" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="
1600" type="4" />
</node>
<node id="ConfigEntry">
<attribute id="MapKey" value="
ScreenWidth" type="22" />
<attribute id="Type" value="0" type="5" />
<attribute id="Value" value="
2560" type="4" />
</node>
