Trim off any large or hard pieces of fat from the lamb shoulder and cut into 3-inch chunks. Transfer to the bowl of a slow cooker.
In a blender combine all remaining ingredients except for bay leaves and blend until smooth (about 1 minute). Pour over lamb and stir to coat. Add bay leaves and cover. Cook on low for 5-6 hours, or until lamb shreds easily with a fork.
When it has cooled slightly, transfer lamb from the cooking juices to a large bowl. Using two forks, shred the meat.
Skim fat off of the juices in the slow cooker; remove and discard bay leaves. Add 1/2 cup or more of the juices to the shredded lamb and gently toss to coat. Refrigerate remaining juices for reheating (see note #2) or freeze for another use (see note #3).
Serve with warm tortillas and your choice of toppings or on tostadas, in burritos, on taco salads.
Notes
Chipotle chile powder is a smoky spice made with only smoked and dried chipotle peppers, not a standard chili powder that is a blend of spices.
My favorite way to reheat lamb barbacoa is in a slick of hot oil in a non-stick or cast iron skillet until the meat is heated through and the edges are crisp, adding a little bit of the cooking liquid for flavor and moisture.
Don't throw the cooking liquid away! If you have some left over, freeze it for a great start for chili or a savory soup.