Stir together flour and oil in a small saucepan over medium heat. Cook, stirring constantly, for 5 minutes. Reduce heat and cook, stirring constantly, for about 10 minutes or until mixture turns reddish brown. Spoon mixture into a slow cooker; stir in remaining ingredients except shrimp and rice. Cover and cook on high setting for one hour; then 5 hours on low setting. Twenty minutes before serving, add shrimp to slow cooker; mix well. Cover and cook on low setting. Ladle gumbo over cooked rice in soup bowls.