Pat the ham dry with paper towels, remove any rind or skin if present, and score around the bone at the narrow end. Place the ham flat side down in the slow cooker insert, trimming the narrow end if needed to fit.
In a small bowl, combine the brown sugar, chili powder, garlic powder, onion powder, smoked paprika, and black pepper, then stir in the stone ground mustard to form a thick, paste-like rub.
Spread the spice rub all over the exposed surface of the ham, pressing it into any crevices, then drizzle the apple cider vinegar directly over the top.
Pour the water into the bottom of the slow cooker around the ham, cover with the lid, and cook on LOW for 7 to 9 hours — 7 hours for a 6 lb ham, up to 9 hours for an 8 lb ham — without lifting the lid during cooking.
Insert a meat thermometer into the thickest part of the ham, avoiding the bone — it is ready to shred when the internal temperature reaches 200–205°F (93–96°C). If it has not reached temperature, continue cooking in 30-minute increments.
Using tongs, carefully transfer the ham to a large cutting board, tent loosely with foil, and let it rest for 10 to 15 minutes. While it rests, skim excess fat from the braising liquid and reserve the liquid.
Using two large forks or bear claw shredders, pull the meat apart along the grain into long shreds, discarding any large pieces of fat, cartilage, or bone as you go. Return the shredded ham to the slow cooker and toss with the reserved braising liquid.
Stir in 1 cup (240 ml) of BBQ sauce, set the slow cooker to WARM, and let the meat absorb the sauce for 10 to 15 minutes before piling onto toasted buns with pickles and diced onion.