Cut onion into quarters then separate layers into the bottom of a 6 quart crock pot. Trim pork butt of excess fat then cut into 4-6 large pieces and season well with salt and pepper. Lay seasoned pork on top of onions then add chipotle peppers, adobo sauce, BBQ sauce, brown sugar, and Dr. Pepper. Cover and cook on low for 8 hours or until pork shreds easily with 2 forks.
Remove pork pieces from the crock pot then shred. Place inside a large bowl then add enough cooking liquid to moisten. Scoop pork onto tortillas then scoop Cherry-Peach Salsa on top and serve.
For the Cherry-Peach Salsa:
Combine all ingredients in a bowl and mix well. Can be made up to two days in advance.