Whisk together the sour cream, mayonnaise, milk, and cumin. Set aside. Skewer the shrimp. (If using wood skewers, soak them in water for 10 minutes beforehand.) In a small bowl, combine the butter and garlic. Preheat a gas grill to high; adjust to medium after 15 minutes. (If cooking over charcoal, allow the coals to burn until they are covered with gray ash.) Brush the skewered shrimp with the garlic butter. Place them on the grill with the limes. Cook about 4 minutes on each side or until the shrimp are opaque and the limes are browned. Remove from grill. Lightly salt the shrimp. Grill the tortillas for 30 seconds on each side, then place inside a paper bag to keep warm. To serve, pull the shrimp off the skewers and divide them evenly among the tortillas. Top with the cabbage, sour cream sauce, tomatillo salsa, and a spritz of grilled lime.
Rainy-Day Method: Broil the shrimp and limes about 4 inches from the heat using the cooking times above. Wrap the tortillas in foil and heat in a 350° F oven for 15 minutes, or wrap them in a napkin and microwave for 3 minutes.