Rub the steak with chipotle powder and 1/2 teaspoon salt. In a large resealable plastic bag, mix 3 tablespoons of the lime juice and the oil. Add the steak, seal the bag and marinate 15 to 30 minutes. Meanwhile, in a medium bowl; mash the avocado with remaining 1 tablespoon lime juice, the cilantro, jalapeno, and 1 teaspoon salt. Cover and refrigerate. Heat grill to high. Just before serving, remove steaks and pat dry; discard marinade. Grill steaks to medium-rare, about 2 minutes per side. On a board, thinly slice the steak against the grain; serve on tortillas, topped with guacamole.