In a large skillet, heat the canola oil over medium heat. Add the chicken and brown on both sides for about 5 minutes per side.
Add the diced tomatoes, onion, cumin, salt, and cayenne. Stir to combine and bring to a boil.
Reduce the heat to low. Cover the skillet and cook until the chicken reaches an internal temperature of 180°F, about 25-30 minutes, stirring occasionally to prevent sticking.
Remove the chicken from the skillet and set aside to cool slightly. Once cool enough to handle, shred the meat using two forks, discarding the bones.
Return the shredded chicken to the skillet with the sauce. Simmer uncovered until the sauce thickens slightly, about 8-10 minutes.
Meanwhile, peel and slice the avocado. Toss it gently with lime juice to prevent browning.
To assemble, lay out a whole wheat tortilla. Spoon 1/2 cup of the chicken mixture onto the center, top with avocado slices, and sprinkle with fresh cilantro, if desired.
Roll up tightly, serve immediately, and enjoy!