Preheat oven to 300 degrees.
Peel onion and cut into 4 pieces. Lay them in the bottom of a large dutch oven.
Add the spice rub to the pork - work it into the meat, then set it on top of the onions in the pan.
Add the chipotle peppers over the pork (include the sauce.) Add in can of Dr Pepper. Add brown sugar.
Place lid on pot, then set pot in the oven. Cook for six hours. Check meat every two hours and use tongs to flip it . After six hours; it should be falling apart, use two forks to see if it shreds easily. If it's not falling apart, return to the oven for another hour or so.
Remove meat from pot and place on a cutting board or other work surface. Use two forks to shred meat. Discard fat pieces. Return the shredded meat to the cooking liquid, and keep warm until ready to serve.