Combine the brown sugar, paprika, salt and pepper, and rub the mixture over the roast. Wrap tightly in plastic, and refrigerate over night.
In a bowl, combine the vinegars, Worcestershire sauce, red pepper flakes, sugar, mustard, garlic powder, and ancho chili powder. Mix well.
Unwrap the roast. I like to line my crock pot with banana leaves (totally optional) Place rough cut onions on the bottom of the crock pot or on top of banana leaves.
Place roast on top of onions, drizzle most of the vinegar mix over the top, reserving some to add to the shredded meat at the end.
Cover and cook on low for 7 to 8 hours, or high for 4 to 5 hours.
Remove the meat and onions to a cutting board. Remove skin and set aside. Using two forks, pull and shred the pork. Chop the onions, and mix into the shredded meat. Using a fork, remove some of the fat from under the skin, mince, and add to the shredded meat and onions as needed for moisture and flavor.
Serve on warm buns or crusty hard rolls, with the remaining vinegar mixture on the side.