In a bowl, cover the sun-dried tomatoes with hot water and let stand for 5 minutes. Drain, pat dry and finely chop.
In a small bowl, combine the thyme, marjoram, tarragon, cayenne and 1/4 teaspoon of salt.
In a large skillet, melt the butter in the olive oil over moderate heat. Add the garlic and cook until fragrant, about 30 seconds. Add the fresh and sun-dried tomatoes and the herb mixture and cook for 2 minutes, stirring. Add the shrimp and cilantro and cook until the shrimp are just heated through.
Fill the taco shells with the shrimp mixture and top with the diced avocados. Alternatively, fill the warmed tortillas with the shrimp and avocados and fold in half. Serve the tacos at once, with salsa and lime wedges.