In a 9- by 13-inch baking dish, combine soy sauce, Merlot, white wine, garlic, chopped oregano, chopped rosemary, and pepper. Add lamb and turn to coat with marinade. Cover and chill at least 6 hours or up to 1 day, turning meat over several times.
Lift lamb from marinade (reserve marinade) and place on a barbecue grill over a solid bed of medium coals or medium heat on a gas grill (you can hold your hand at grill level only 4 to 5 seconds). Close lid on gas grill. Cook, turning as needed to brown meat evenly, until a thermometer inserted in thickest part of leg reaches 135° to 140° for rare (thinner portions will be well-done), 30 to 45 minutes. Brush meat occasionally with marinade up until the last 10 minutes of cooking.
Transfer meat to a platter, keep warm, and let rest 5 to 15 minutes. Garnish with oregano and rosemary sprigs. To serve, thinly slice meat. Add salt to taste.