My credit score is only around 640. I really want to refinance my truck and get away from 15% interest.?

I bought this truck exactly 1yr. ago. I owe 27,000. The payments are high but I have made them all, on time. I just hate paying this much interest. Is there any place I can go that is an almost "surefire" approval? I don't want to try 3 or 4 places and bring my score down even more, and I bank at a bank that is not known for working with people with this type of score. The truck was financed through the dealership at Capitol One