http://www.glorecords.blm.gov/ Hi Chris, was just over at Federal Land Office and thought you might want to take a look. I found 6 listings for land for Riddicks in Arkansa from1860 to 1881 and about 20(or more)listed for Darby in Arkansas. The link I am sendin will take you to there main page,then just click on "search land patents" tab at top of page. Not sure if these are any of your line but thought I might pass it on.
I wasn't really sure if they were any of you ancestors or not. But thought I would send it just incase they may be. Did not find any for La. under Riddick surname. But did find quite a few for Darby . I will let you know if I should come accross anything else.