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Rinse the pork well in cold water. Trim off any excess fat using a sharp knife. Roughly slice the onions. Place one sliced onion in the bottom of the crockpot. This will prevent the meat from burning. Place the roast on top, then cover with the second onion. Pour the ginger ale over the pork and onions. Cover the pork and cook on low for approximately 12 hours. Depending on the size of your pork and the type of crockpot, you may need to adjust this time upwards or downwards. When it's cooked, you should be able to shred the pork easily with a fork. If you cannot, it probably needs more cooking time. When the meat is tender, remove the pork from the crockpot and drain the liquid. Set the onions aside for now. Using two forks, shred the meat, discarding any fat, bones or skin. Most of the fat will have melted away by now. Place the shredded meat and onions back in the crockpot. Stir in the entire bottle of barbecue sauce. Continue to cook on low heat for another 4-6 hours. Serve your barbecue pulled pork with hamburger buns or rolls and additional barbecue sauce. Any leftovers will keep well in the freezer.

Summary:
Prepare the ingredients. Assemble the ingredients in the crockpot. Begin cooking. Drain and shred the meat. Return the meat to the crockpot. Serve.