Prepare the spice mix by combining the spice ingredients in a small bowl.
Once the bulk of the fat has been trimmed from the shoulder, cut into thick strips, and give a very liberal dusting of an all-purpose rub.
Place the meat, onion, garlic, whole ancho pepper, thai chilies in a crockpot / slow cooker, dust with half the spice, and toss to mix it all up.
Sprinkle the rest of the spice mix on top and give another toss.
Pour the juice in and set the crockpot to cook for 8 hours on low or 5 hours on high.
Once complete, remove the meat from the pot and pour the juices into saucepot.
Bring to a boil, and reduce, boiling for about 5-8 minutes.
Shred the meat with 2 forks.
Pour the sauce over the meat until its your desired wetness.