Add the pork shoulder chunks, sliced onion, and minced garlic to the slow cooker.
Sprinkle the cumin, oregano, and chili powder over the pork. Pour in the orange juice and chicken broth. Season with salt and black pepper.
Cover and cook on low for 8 hours until the pork is very tender.
Remove the pork from the slow cooker and shred it with two forks. Return the shredded pork to the slow cooker and stir to coat it in the juices.
Divide the cooked rice among four bowls. Top each bowl with the shredded pork and some of the onions from the slow cooker.
Add chopped cilantro, lime wedges, and sliced avocado to each bowl before serving.