Trim fat from meat. In a small bowl stir together chile powder, salt, and pepper. Sprinkle mixture evenly over all sides of meat; rub in with your fingers.
For a charcoal grill, arrange medium-hot coals around a drip pan. Test for medium heat above pan. Place meat on grill rack over drip pan. Cover; grill about 4 hours or until meat is very tender. Brush onion slices lightly with oil; place on grill rack directly over coals the last 20 minutes of grilling, turning once. (For a gas grill, preheat grill. Reduce heat to medium. Adjust for indirect cooking. Grill as above, except place meat on a rack in a roasting pan.) Remove meat from grill. Cover with foil and let stand for 30 minutes.
Shred pork with two forks. Spread some of the Chipotle Mayonnaise on one side of each tortilla. Top each tortilla with meat, grilled onion slices, and Montery Jack cheese. Tightly roll up each tortilla. Makes 8 to 10 wraps.
In a small bowl stir together mayonnaise or salad dressing and chipotle peppers. Cover and chill until serving time or up to 1 week. Makes 1 cup.