1. Slice onions and place them in the bottom of the slow cooker.
2. Combine brown sugar, cayenne, garlic, chipotle, salt, pepper, and minced jalapenos in a small bowl to make a dry rub.
3. Rinse the pork butt and pat dry with a paper towel.
4. Rub the spices onto the pork and place in the slow cooker on top of the onions.
5. Pour the Dr. Pepper down the side of the slow cooker.
6. Cook on Low for 7-8 hours or high for 5-6 hours (or until internal temp is 160).
7. Remove pork from slow cooker and allow pork to rest on a cutting board for 10 minutes. Make sauce while meat rests.
8. Remove onions and set aside to top pork, or dice them to include them in the pork shreds.
9. Reserve 1/2 cup of liquid from the bottom of the crockpot for the sauce, and discard the rest.
10. Shred pork with 2 forks and return it to the crockpot.
11. Pour sauce (see recipe below) over the top and stir to coat shredded pork.

For Sauce

1. Combine sauce, liquid from the crockpot, and 1 cup Dr. Pepper in a small saucepan over medium low heat.
2. Simmer until sauce has thickened again, about 10-15 min.