An optional step I highly recommend. It can be done directly inside the slow cooker or multicooker if you have that function. If not, brown them in a large skillet by heating heat the oil over medium-high heat. Brown the ribs in batches, 3 to 4 minutes per side. Transfer the browned ribs to the slow cooker.
Cook:
In a blender, blend the chipotle peppers and adobo sauce, water, barbecue sauce, lime juice, vinegar, garlic, cumin, and salt until smooth. Pour the thick sauce over the ribs. Reserve one-quarter of the chopped onion and put the rest in the slow cooker.
Cover and cook on high for 6 hours or on low for 10 hours, or until the meat easily separates between two forks.
In the last couple of hours, use tongs to flip the ribs and distribute the sauce on top of the ribs. When the meat is done, break it up into smaller pieces with two forks.
Assemble the tacos:
To assemble the tacos, start with a base of meat on each tortilla, top with some of the reserved chopped raw onion, pico de gallo, sliced avocados, sour cream, and cilantro. Serve with grated cheese and lime wedges on the side if you desire.
Notes
If you can't find chipotle peppers in adobo sauce you can substitute ¾ cup of enchilada sauce.