Make the avocado dressing first. Add the avocado, sour cream, mayonnaise, garlic, cilantro, lime juice, and salt to a blender or food processor. Blend until smooth. Set the dressing aside.
Finely shred the cabbage, so the slaw is ready to be mixed just before serving. Thinly slice the jalapeño (if using).
Toast the corn tortillas. You can do this over the open flame of a gas stove, or in a very hot dry skillet. When the skillet is very hot, the tortillas only need to touch the skillet for about 10 seconds on each side to give it that toasty corn flavor. This also helps strengthen the tortillas a bit.
If you shrimp are frozen, place them in a colander and run cool water over top until thawed. Remove the peel and tail, if attached. Allow the shrimp to drain well, then place them in a bowl and add the Tajín. Stir to coat the shrimp in Tajín.
Heat the cooking oil in a large skillet over medium. When the skillet and oil are very hot, add the seasoned shrimp and sauté until the shrimp are opaque (this should only take about three minutes). If there is a lot of liquid in the bottom of the skillet, remove the shrimp and then allow the liquid to simmer until thickened. Turn off the heat and then toss the shrimp in the thickened sauce.
Just before serving, combine the shredded cabbage with about ½ cup of the avocado dressing and stir until the cabbage is evenly coated. The rest of the avocado dressing will be drizzled over the tacos.
To build the shrimp tacos, add a handful of slaw to the bottom of each tortilla then top with a few shrimp and a couple slices of jalapeño. Drizzle a little of the remaining avocado dressing over top, then enjoy!