Bring a saucepan of salted water to a boil. Fill a bowl with ice water. Blanch the shrimp just until white throughout, 3 minutes. Drain and chill the shrimp in the ice water, then drain again and pat dry. Transfer the shrimp to a large bowl. Add the romaine, tomatoes, cucumber, cilantro, avocado and tortilla chips.
In a small bowl, whisk the sour cream with the mayonnaise, lime juice and lime zest and season with salt and pepper. Pour the dressing over the salad and gently toss to coat. Serve right away.