Mix the BBQ rub ingredients together and evenly coat the trimmed pork loin. Set aside.
Preheat your smoker to 225ºF* for 15 minutes, lid closed.
Add the pork loin to the grill and cook until it reaches an internal temperature of 155ºF to 165ºF or until you're happy with the bark that's formed, about 4 hours. Optional: Spritz with apple juice or apple cider vinegar every 60-90 minutes.
Mix the apple juice and BBQ sauce together in an aluminum grill pan or smoker-safe dish. Place the smoked pork loin in the tray and cover tightly with foil.
Increase the temperature to 300ºF and smoke until the pork reaches an internal temperature of 205ºF in several places.
Let the pork rest for at least 30 minutes before transferring the sauce in the pan to a bowl and pulling the pork. You can add the sauce back to the pork or serve it on the side. (To thicken the sauce, stir in a slurry consisting of 2 Tablespoons corn starch and 1 Tablespoon water.)
