To make the sauce, pit and peel the avocado. Place in a food processor or blender with the cucumber, sour cream, green onions, dill, Tabasco sauce, lime juice and water. Puree until smooth. Season with salt and red pepper to taste.
Combine the rub ingredients in a bowl and toss with shrimp to coat.
In a medium non-stick skillet, melt the butter over medium heat & saute the shrimp and garlic until the shrimp turns pink and is slightly browned.
Remove from heat and sprinkle with the cilantro. Fold the tacos and serve with lettuce and tomato.