Place ranch dressing in a resealable plastic bag. Add chicken; seal bag, turning to combine. Refrigerate at least 4 hours or overnight. (I recommend overnight).
Preheat oven to 200°. In a 12-inch cast-iron grill pan (or any skillet pan will work), heat oil over medium-high heat. Add chicken and cook until golden brown and cooked through, about 4 to 6 minutes per side. Transfer chicken to a baking sheet, and place in preheated oven to keep warm until you've cooked all the tenderloins and are ready to serve them.
In a small bowl, whisk together lime juice, mayonnaise, sour cream, sugar, vinegar, and salt. In a large bowl combine coleslaw mix, jalapeño, and cilantro. Add dressing; toss to combine.
Place a chicken tender on a tortilla and top with coleslaw mix, cumin sour cream and other desired toppings.
You may or may not use all the coleslaw mix, depending how how "wet or dry" you want your coleslaw. I would start out with less and mix in the dressing and then see if you want to add more cabbage or not.
I love to buy the prepackaged coleslaw rather than buy cabbage and have to slice it myself. It's definitely much faster and easier to buy it in the package.
You most likely will not use all the coleslaw on top of the tacos, but we just ate the extra as a side dish.
I recommend marinating the chicken tenders overnight. It makes them even better and more tender.