Cut onion into quarters then separate layers into the bottom of a 5 quart (minimum size) crock pot. Trim pork butt of excess fat then cut into 4-6 large pieces and season well with salt & 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 is tender enough to easily shred with a fork.
Remove pork pieces from the crock pot and shred, then place into a bowl and set aside. Strain juices through a double layer of cheesecloth and add back to shredded pork.
Serve with Cherry-Peach Salsa in flour tortillas.
For the Cherry-Peach Salsa: Combine all ingredients in a bowl and mix well. Make up to two days in advance.