Remove the netting from the pork shoulder roast and pat the pork dry with paper towel. Place into a large baking dish or tray to catch any mess while seasoning. Optional - Remove skin from pork - see Notes below.
Combine the brown sugar, smoked paprika, garlic powder, salt, oregano, black pepper, mustard powder, onion powder and cumin in a small bowl.
Sprinkle the spice rub generously all over the pork, rubbing it into all the nooks and crannies until well coated.
Transfer the pork to the slow cooker along with all the leftover spices from the baking dish. Spoon over the diced chipotles in adobo sauce and gently spread over the surface of the pork.
Cover and cook on LOW for 8 to 9 hours or until the pork is tender and easily shreds with two forks. There is no need to add extra liquid as the pork will release plenty of juices while cooking.
Carefully transfer the cooked pork to a clean baking dish or tray. Remove and discard any skin, if you did not do this step earlier, and any large pieces of visible fat.
Shred the pork using two forks or kitchen tongs until pulled into thick strands.
Return the shredded pork to the slow cooker and stir through the cooking juices.
Serve as is or stir through BBQ sauce to taste for a sweeter BBQ-style pulled pork. If needed, cook on HIGH for 30 minutes to heat everything through before serving.