Make the short ribs
Light about 25 charcoal briquettes in a chimney and prepare the grill with the coals on one side only. Rub the ribs with 2 Tbs. of the spice rub and place them in a pan. Put the pan on the grill over medium-hot coals and turn the ribs when they start to sizzle. Continue cooking in the dish for 1 hour, turning to caramelize on all sides. Move the pan to the cool side of the grill if the meat begins to burn or stick.
Tear the ancho chiles into pieces and combine with the root beer in a saucepan over medium heat. Simmer until the chiles soften, about 15 minutes. Heat the oil in a skillet and add the onion. Cook until softened, about 5 minutes. Add the garlic and cook for another minute. Add the root beer and chile mixture and bring to a boil, then reduce the heat to a simmer.
Add more wood and charcoal to the grill if needed. When the ribs are well browned, pour the molasses over each rib, turning to coat. Then add the hot root beer–chile mixture to the pan. Place the pan over hot coals so it simmers.
Cover the grill and allow the ribs to smoke and simmer for another hour, turning often. Cover the pan with aluminum foil and allow to steam for 30 minutes. Remove the pan from the grill and put the ribs in a serving dish. Stir the braising liquid and molasses in the pan together and pour over the ribs. Serve immediately with the flour tortillas, sliced onion, cilantro, and refried beans.