In a medium bowl, mix the brown sugar, cumin, chili powder, onion powder, garlic powder, mustard powder, salt, and pepper. Rub the pork with the seasoning mixture until well coated.

Place the pork in a slow cooker and pour the apple juice and 1 cup water on top. Cover and cook on low for 6 to 8 hours, until the pork shreds easily with a fork.

Place the pork on a platter and shred all the meat with two forks. Return the meat to the slow cooker with its juices and keep warm until ready to use (or refrigerate the pork in its juices and reheat when ready).

In a medium skillet over medium-high heat, char the tortillas for 1 minute on each side, until browning begins. You want the tortillas to still be malleable, not hard or crispy.

Layer the pulled pork, Cheddar, onion, cilantro, and salsa on the tortillas and serve.