In a large mixing bowl, combine the brown sugar, cumin, garlic powder, and chili powder with a big pinch of salt and several grinds of black pepper.
Add the pork and rub it evenly with the sugar and spice mixture.
Place the pork in the crock of your slow cooker along with all of the sugar and spice mixture.
Pour in the chicken broth, vinegar, and soy sauce.
Set your slow cooker on low.
Simmer the pork for 8 hoursitll end up really soft and tender.
At this point you can serve the pork in slices, with a little of the cooking juices spooned over the meat.
Or, for a crisp exterior, drain the cooking liquid into a medium saucepan, add the optional 3 additional tablespoons of brown sugar, and reduce by about half, until it forms a dark, shiny glaze.
Place the unsliced pork under a hot broiler until it begins to brown, about 4 to 5 minutes.