Have the truck scanned to make sure you don't have a sensor bad causing the lockup or a bad solenoid in the transmission. The torque converter solenoid may not be operaing correctly and when the fluid is cold it doesn't flow through as quickly. If the fluid looks and smells burnt you have deeper troubles.

That truck will tell you a lot about what is happening through the computer and a scanner.

But be ready, usually if you are having that kind of trouble, there is a deeper underlying cause.