Squeeze limes into measuring cup; reserve 1 tablespoon juice for the salsa. In heavy-duty resealable food-storage plastic bag, mix remaining lime juice (about 1/4 cup), the cumin, 1/4 teaspoon salt, the pepper and garlic. Add beef; seal bag and turn to coat beef. Refrigerate 1 hour to marinate, turning occasionally.
Meanwhile, in medium bowl, mix tomatoes, onion, avocado, reserved 1 tablespoon lime juice and 1/8 teaspoon salt; toss to coat.
Set oven control to broil. Remove beef from marinade; discard marinade. Place beef on rack in broiler pan. Broil with top 4 to 6 inches from heat 10 to 15 minutes or until desired doneness, turning once. Let stand 5 minutes. Cut diagonally across grain into very thin slices.
Warm tortillas as directed on package. Divide steak strips among tortillas; top each with about 1/4 cup avocado salsa; fold in half.