In a small bowl, blend oil, vinegar, salt and garlic. Slice 4 of the shrimp lengthwise; cut remaining shrimp into 1/2 inch pieces. Add shrimp to oil mixture and set aside.

Halve avacados lengthwise and remove pits. With a spoon, carefully scoop out bite-size chunks of avacado and add to shrimp mixture, stirring to coat well; reserve avacado shells. If made ahead, cover avacado mixture and shells and refrigerate for up to 6 hours, gently blending mixture once or twice.

Set aside shrimp halves and fill shells with remaining avacado mixture; then top each with 2 shrimp halves. Sprinkle with pistachios.

Line 4 salad plates with lettuce leaves and arrange a filled shell on each.