Reserve ¼ cup of the Green Chile Marinade. Add the pork roast to a slow cooker and pour the remaining marinade over the top. Cook on low for 8 hours.
In a food process or blender, combine the sour cream, avocado, and reserved marinade. Blend until smooth and creamy. Chill until ready to serve.
Transfer the pork roast to a cutting board and shred or roughly cut. Add pork back into the cooking liquid to absorb the green chile marinade.
Serve with warm corn tortillas, sour cream sauce, cilantro and diced onion.