In a large slow cooker pot, add water, apple cider vinegar, and the pork roast.
On top of the roast, add half of the following ingredients; Ancho chili powder, Chipotle chili powder, cumin, smoked paprika, salt, pepper, and oregano. With a cooking brush, spread all of the spices evenly on top of the meat. Using a large cooking fork, turn the pork roast over on its other side and top with the remaining spices. With a cooking brush spread all of the spices evenly on top of the meat.
Top the pork with onions and garlic.
Cover the pot with a lid and cook on the low setting for 6 hours or until the pork is moist and tender.
When the pork is moist and tender, transfer it to a large shallow dish such as a casserole dish and reserve the liquid that is left in the slow cooker.
With a fork in each hand, pull the pork apart into pieces. Add a couple ladles of the liquid left in the slow cooker and mix it into the shredded pork. Salt to taste and serve warm.