Bring a large pot of salted water to a boil. Add rotini and cook according to package directions until al dente. Drain and rinse under cold water to stop cooking and cool quickly.
In a large mixing bowl, whisk together mayonnaise, sour cream, pickle juice, garlic powder, salt, and pepper until smooth and well combined.
Add the cooled pasta, chopped pickles, red onion, and fresh dill to the bowl with the dressing. Gently fold everything together until evenly coated.
Cover and refrigerate for at least 1 hour (overnight is ideal) to allow flavors to meld. Before serving, stir again, adjust seasoning, and garnish with extra dill if desired.