Dredge lamb shanks in flour, coating well. Sauté shanks in shortening in a heavy skillet over medium heat until browned. Drain shanks on paper towels.
Place shanks in a lightly greased 2 1/2-quart shallow baking dish. Sprinkle shanks with salt and pepper. Combine sugar, water, vinegar, catsup, Worcestershire sauce, onion, raisins, and prunes; mix well. Pour sauce mixture over shanks. Cover and bake at 300° for 2 hours or until shanks are tender.