Is there any way I can force just my shouts on Foursquare to be shared on Facebook/Twitter? All other checkins (off the grid or otherwise) should not be shared.
Are you on foursquare.com? Or a mobile device?
On Android, just open the foursquare app, open the "Me" tab and press menu button (hardware menu button) and click "Shout." This way you'll be able to only shout, and not check in. If you chose to shout along with a check it, it's going to send the location/venue info because you checked in to it.