Preheat grill to 350 degrees. Season chicken with salt and pepper. Cook chicken until done, about 5-7 minutes per side. (Alternatively, cook chicken in a 375 degree oven for 20-25 minutes). Remove onto a work surface. Slice the chicken into thin strips. Set aside.
Make the chipotle mayo: Finely chop the adobo chiles. Combine chiles, mayonnaise and lime juice in a small bowl. Set aside.
Make the avocado ranch sauce: Mash the avocado in a small bowl. Pour in the ranch dressing and mix to combine.
Assemble the burritos: Smear a line of chipotle mayo down the center of a tortilla. Arrange sliced chicken, black beans, rice, lettuce, tomatoes, cheese and tortilla strips over the chipotle mayo in the center of the tortilla. Be careful not to go all the way to the ends of the tortilla - leave 1/2 inch perimeter.
Fold one end of the tortilla over the fillings, using your hands to gently tuck and press the tortilla firmly around the fillings. Fold the ends over the fillings. Continue to roll the burrito, setting it seam side down on a work surface.
Grill the burritos: Coat a nonstick pan with a thin layer of oil. Heat the pan over medium heat. Place a burrito, seam side down, onto the pan. Grill until golden brown. Flip. Once both sides are golden brown, remove from pan.