I agree - they can't tell you how to drive the truck. As long as it's under redline, what's the problem? Also, ride with the mechanic (if you didn't) and make sure he's driving it like you do. It's not a race car, but it is a performance street truck and it should drive like one. If they won't fix it, take it to the next closest dealer and explain that the dealer you bought it from refuses to acknowledge or fix the problem.
DON'T try to mess it up more. That's bad advice and will only lead you down a bad road. Be truthful - it will get you the furthest.
Good luck bro.