Preheat oven to 325°F. Wrap tortillas in foil and place in oven.
Halve and pit avocados. Score flesh with a knife to make small, diced pieces and then scoop pieces into a bowl. Squeeze in juice from lime quarters, add diced tomatoes and salt and toss mixture gently. Set aside.
Finely chop onion and put pieces in a small bowl. Slice lettuce into small, thin ribbons and put in a separate bowl.
Heat oil in a heavy skillet over medium-high heat until hot but not smoking. Add prepared shrimp and sprinkle chili powder and cumin on top; cook, stirring, until shrimp is pink and curls up, 2 to 3 minutes. Transfer cooked shrimp to a bowl. Remove tortillas from oven.
Serve warm tortillas with shrimp, lettuce, onion, and avocado-tomato mixture in serving dishes.