I have a 1 year old 2008 3500 dually with 4500 miles on it. When the truck is cold (sitting all day) and I head for home, the shift into 6th will delay sometimes for 2 miles. Maybe the lock up? Speed 70 mph and RPM about 2600. Do I need to let the engine warm up longer or drive slower or what? Never happens on my 5. 9 auto transmission.