I live in the desert and can't always fire up the oven, so I make this in a crock pot. Here's what I do:
Layer, in order:
1 large onion cut into wedges
1 pork shoulder (a.k.a. pork butt) 5-7 pounds
11 oz can chipotle peppers in adobo sauce
2 cans dr pepper
2 Tb brown sugar
Cook on low until falling apart.
Shred meat, discard fat. Spoon as much fat of the liquid as possible. Return meat to crock pot to keep warm.
Serve in tortillas w shredded lettuce, salsa, guacamole, cheese, whatever.