Preheat grill to medium-high.
Mash avocado with a fork in a small bowl. Add lime juice, garlic and salt and stir to combine.
Pat shrimp dry. Toss the shrimp with Cajun seasoning in a medium bowl. Thread onto four 10- to 12-inch metal skewers. Grill, turning once, until the shrimp are just cooked through, about 4 minutes total.
Serve the shrimp in tortillas, topped with the guacamole, lettuce, cilantro and pico de gallo.