In bowl, combine half of the lime juice, the chipotle pepper, adobo sauce, oregano, cumin, coriander, salt and pepper. Add pork, turning to coat. Cover and refrigerate for 1 hour. (Make-ahead: Refrigerate for up to 24 hours.)
Peel, pit and chop avocado. In bowl, mash avocado with fork; mix in cilantro and remaining lime juice. Set aside.
Weave pork slices onto 4 metal or soaked wooden skewers. Place on greased grill over medium-high heat; close lid and grill, turning occasionally, until juices run clear when pork is pierced and just a hint of pink remains inside, 8 to 10 minutes. Remove from skewers.
Layer bottom half of each bun with lettuce, tomatoes, pork and onion. Spread avocado mixture on cut side of top of each bun; sandwich layers with tops.