Bring 3 quarts water and crab seasoning to a boil in a Dutch oven; add shrimp, and cook, stirring occasionally, 2 minutes or just until shrimp turn pink. Drain and cool.
Peel cooked shrimp, and devein, if desired; coarsely chop shrimp.
Stir together mayonnaise and next 5 ingredients in a large bowl; stir in shrimp. Chill 2 hours.
Stir together butter and garlic, and spread evenly on cut side of bread halves. Bake at 375° for 10 minutes or until toasted.
Line each roll with a lettuce leaf, if desired. Spoon shrimp mixture evenly on each roll. Top with avocado, if desired.