Combine marinade ingredients in a large dish and add skirt steaks. Cover and let marinate in the refrigerator for 2 hours. Turn 2 or 3 times. Remove from marinade, drain, and pat dry. Brush steaks with olive oil just before grilling.
Combine all the glaze ingredients in a small saucepot and place over medium heat.
Reduce heat and let simmer for 30 minutes. Remove from heat and let cool. Pour off half of glaze recipe into a bowl for brushing, and reserve the other half in a separate bowl to prevent cross-contamination.
Preheat the gas grill or briquettes to medium heat.
Brush steaks with olive oil and grill over an open fire for about 3 minutes on one side. Turn once and brush over half of the guava glaze. Cook another 3 minutes or until reaching desired doneness. Remove from heat. Let rest 5-7 minutes. Slice thinly, across the grain, and place in a medium-size mixing bowl. Season glazed cut steak with kosher salt. Spoon the remaining reserved glaze over the top of the cut steaks and toss to coat. Serve over tostadas, salad or tacos.