Myth about warming up ur car before driving it.

An Idle motor on a fuel injected vehicle is virtually the same thing as the choke used to be for a carburated engine. It is basically a solenoid valve that adjusts the air fuel mixture to be rich while an engine is warming up to operating temperature. Once the engine is warm the idle motor is taken out of the mix and the computer begins to control the air fuel mixture. And no I don't believe it would have any effect on your truck shifting into or out of overdrive. Sounds more like torque converter clutch shutter.

Check the operation of the coolant temp sensor.
Check the thermostat is also working properly.

check and or clean the (IAC) idle air control, this is what controls idling on most of todays cars and light trucks. if iac is removed for cleaning. clean out where it mounts to engine or fuel injection unit. good-day !

Check temperature sensor. I don't remember if your vehicle has two (one for the gauge, one for the computer) or a single unit that feeds both. If seperate (your parts house should know this) change the one that feeds info to the computer.
Essentially this sensor does the job of the choke on the carburetor on non-fuel injected vehicles, by allowing additional fuel into engine on cold-starts. After engine comes up to operating temp, it reverts back to normal run mode... (why you need to warm it up at excessive rpm before driving!) Good luck!

The high idle is caused by the choke and the high idle on the carb. It is working normal and you do won't to warm up this motor before going. When it is warm and you hit the gas it activates the choke pull off to bring the idle to normal. Your mechanic is used to fuel injection that will control the fuel/air mix according to temp. I still let my injected cars warm up so the oil can thin a little. If you jump on the highway with cold thick oil it can all wind up in the to of the motor.