Lime juice and adobo sauce lend a tangy and smoky flavor to these grilled steak tacos.
Combine oil, 2 Tbsp. lime juice, adobo sauce, brown sugar, garlic, and 1/4 tsp. salt in a small bowl. Transfer 2 Tbsp. of the mixture to a separate small bowl. Stir in yogurt; cover and refrigerate.
Place steak in a glass dish and pour the remaining adobo mixture over the steak, turning to coat evenly. Cover and refrigerate for at least 2 hours or up to 12 hours, flipping halfway through.
Combine avocado, tomato, cilantro, onion, jalapeño, and the remaining 2 Tbsp. lime juice and 1/4 tsp. salt in a medium bowl; toss to combine.
Preheat grill to medium-high. Lightly oil the grill grates (see Tip). Remove the steak from the marinade (discard the marinade). Grill the steak until an instant-read thermometer inserted in the thickest part registers 125 degrees F for medium-rare, about 4 minutes per side. Let rest for 5 to 10 minutes. Thinly slice the steak against the grain.
To assemble: Divide the steak among the tortillas. Top each with a generous 1 Tbsp. yogurt sauce, about 1/3 cup avocado salsa, and 1/4 cup lettuce.
To make ahead: Prepare marinade and sauce (Step 1) and refrigerate for up to 1 day. Marinate steak (Step 2) for up to 12 hours.
Tip: To oil grill rack: Rub an oil-soaked paper towel over the grates (use tongs to hold the paper towel). Do not use cooking spray on a hot grill.