Toss in the chopped onion, can of chipotle peppers in adobe sauce, brown sugar, salt, and pepper.
Pour in 2 cans of Dr. Pepper soda pop, holding the bag up to prevent spills.
Place in the crockpot. Cook on high for 4-5 hours or low for 7-8 hours.
Thaw.
Place bag contents in the Instant Pot and set to sealing. Cook on Manual for 60 minutes. Natural release.
Shred and serve on buns.