Place pork in a slow cooker with enough water to cover bottom by 2 inches. Season pork with garlic salt and pepper.
Cook on High for 4 hours.
Preheat oven to 350 degrees F (175 degrees C).
Transfer pork to a bowl and shred meat with 2 forks. Mix 1 can enchilada sauce and brown sugar into pork. Pour half the remaining can of sauce into a 9x13-inch baking dish.
Spread about 1/4 cup pork mixture down the center of each tortilla; top with black beans and cilantro. Roll tortillas around filling and arrange in prepared baking dish, seam sides down. Pour remaining half can enchilada sauce over tortillas and cover with Monterey Jack cheese. Cover baking dish with aluminum foil.
Bake in the preheated oven until cheese is melted and sauce is bubbly, 25 minutes.
