Preheat oven to 300 degrees.
Generously salt & pepper the pork roast, then set it on top of the onions in your covered roaster.
Pour the can of chipotle peppers over the pork (include the sauce). Pour in both cans of Dr. Pepper. Add brown sugar to the liquid & stir in.
Place lid tightly on pot, then set pot in the oven. Cook for at least 6 hours, turning roast 2-3 times during the cooking process. Check meat after six hours; it should be falling apart. Use 2 forks to test. If it doesn't fall apart, return to the oven for an additional hour.
Remove meat from the pot, and place on a cutting board of other work surface. Use 2 forks to shred meat, discarding large pieces of fat. Strain as much of the fat off the top of the cooking liquid as you can & discard it. Return the shredded meat to the cooking liquid, and cook for another 30-60 minutes.
Alternatively, you can cook this in the crockpot on low for 7-8 hours - shred, then cook another 30 to 60 minutes.