Start by preheating your grill or grill pan to high heat.
Mix the brown sugar, chili powder, cumin, paprika, and lime zest in a medium-sized bowl.
Add the shrimp to this bowl and toss everything together until they're well-coated.
Allow them to marinate for about 10 to 15 minutes.
In another bowl, mix the sour cream with a couple teaspoons of the sugar-spice combination you prepared earlier. Set it aside for later use.
Grill the shrimp for 2 to 3 minutes on each side, or until they're cooked through and slightly charred.
Remove them from the grill and set them aside.
Assemble the tacos by placing a few shrimp in each tortilla, topping them with the creamy sauce, and serving.