Bring a large pot of salted water to a boil and cook the rotini about 30 seconds short of the package time. Drain, then rinse under cold water until cool. Drain well so the dressing does not thin out.
Dice the pickles, cheddar, red onion, and celery into small, even pieces. Chop the dill just before mixing so it stays bright.
In a small bowl, whisk the mayonnaise, sour cream, pickle juice, garlic powder, pepper, and a pinch of salt until smooth. Taste it, and add more pickle juice or salt if you like.
Add the pasta, pickles, cheddar, onion, celery, and most of the dill to a large bowl. Pour in the dressing and fold gently until evenly coated.
Cover and refrigerate for at least 1 hour so the flavors blend. Before serving, stir, taste for salt, and add a splash of pickle juice if the salad looks dry. Top with the remaining dill.
