Hello and welcome to JustAnswer!
I'm assuming the check engine light doesn't come on and there are no fault codes stored. If the light is on and/or there are codes stored then that would be the place to start to diagnose the problem.
It sounds like you've got a bad throttle position sensor (TPS). The TPS is mounted to the left side of the throttle body and tells the engine controller how far the throttle is open.
When the sensor gets worn it can start to create voltage spikes in it's signal. The engine controller will see these increases in signal voltage and think you are opening the throttle when you aren't. This causes the idle to increase and decrease on it's own.
The transmission controller also gets the TPS signal for shifting control. When the TCM sees these voltage increases while you are driving at a steady speed it will think you are accelerating and the torque converter clutch will unlock. This will give you an RPM increase of a few hundred RPM. In your case it sounds as if it's severe enough that the transmission is downshifting.
Ho, the TPS is different from the idle air control (IAC) motor. The IAC is mounted on the left side of the throttle body just under the TPS. It's job is to let air into the engine when the throttle is closed.
Replacing the TPS isn't major. The sensor will cost around $50 from your dealer, aftermarket will be less. Replacing it only involves unplugging it and removing two screws. It may be something you can do yourself, otherwise you shouldn't pay more than .5 to 1 hour of labor for a shop to do it.
The trans fluid is definitely due for change, but the fluid shouldn't be a factor in this problem.
You're welcome! Good luck with your truck and Happy New Year!