Cook the rotini pasta according to package instructions until al dente. Drain and rinse under cold water to cool.
In a large mixing bowl, combine the cooked pasta, shrimp, imitation crab, cherry tomatoes, celery, red bell pepper, and red onion.
In a separate bowl, mix together the mayonnaise, sour cream, lemon juice, dried dill, salt, and pepper until smooth.
Pour the dressing over the pasta and seafood mixture, and gently toss until everything is well coated.
Cover the bowl with plastic wrap and refrigerate for at least 1 hour to allow the flavors to meld.
Before serving, give the salad a gentle stir and adjust seasoning if necessary.