Dude, i'm not your financial analyst, and I totally get the allure of a new truck, but what you are proposing makes no sense!
Based on what you wrote, you're 4 years into your current loan and truck with about $26K left on the loan.
By trading in your truck, and getting an '18. You're pretty much essentially getting the same truck, but will start another $42K loan that you'll stretch out to 7 years?
That means you're basically driving the same truck for 11 years, and with interest and principal on both loans, paying close to $90K.