Pat the pork roast dry and season with salt and pepper. Arrange the diced onion and garlic on the bottom of the crock pot.
Season the pork with salt and pepper. Place in the crock pot.
Whisk the water, cider vinegar, Worcestershire sauce, and ancho chili powder in a bowl together. Pour over the pork. Cover with lid and allow to cook on low for 8 to 10 hours, until the pork easily pulls apart.
Remove any fat from the pork pull apart with 2 forks. Toss with barbecue sauce and allow to thicken about 30 minutes before serving on fresh buns.