Rub pork butt generously with your choice of dry rub seasoning on all sides. Or, homemade in Cook's note. Wrap with plastic wrap, and refrigerate overnight.
To prepare: Spray the crock of a 6 quart slow cooker with cooking spray.
Mix Dr Pepper with the liquid smoke. Pour into slow cooker. Center the pork shoulder in the cooking liquid.
Cook on the low for 8-10 hours until fork tender.
Remove pork to a platter. Discard the cooking juices.
Shred pork using 2 forks, removing any visible fat. Return pulled pork back to the slow cooker. Increase the temperature to high.
Pour 2 cups barbecue sauce over pork. Cover and simmer 30-45 minutes just until sauce has simmered into pork.
Serve on kaiser rolls or buns with cole slaw and extra sauce on the side.