In a small bowl, combine chili powder, cumin, oregano, salt and pepper. Rub mixture onto the outside of the pork shoulder.
In the slow cooker, combine orange juice, lime juice, garlic and onion. Place pork on the top. Cook on low for 8 hours or high for 4-5 hours
Remove roast from slow cooker and shred. Remove to slow cooker and heat for an additional 30 minutes.
Remove the pork from the slow cooker and place on a baking sheet. Broil in the oven for 4 minutes to lightly brown edges.