In a large oven proof pan combine the cubed pork, stock, orange juice and salsa, this mixture should totally cover the pork, if not add a little water until the pork is submerged. Bring to a boil. Once boiling, reduce heat to medium low and simmer with the lid on for 3.5 hours, at which point the pork should be falling apart to the touch. Remove the lid and let the pork cook for about 20 minutes more until most of the liquid is evaporated. Remove the pan from the heat and shred the carnitas with 2 forks and set aside. Transfer the carnitas to a baking sheet and put into an oven on broil to crisp. (FOR THE INSTANT POT - do 1/4 cup orange juice and 1/2 cup salsa, no stock for 35 minutes in an instant pot. Then let de-pressurize for 20 minutes before opening.