Before you start with the big stuff, start small bc that's often where the problems lie (and what a lot of mechanics skip). You wouldn't believe how many mechanics skip basic troubleshooting, looks for the 'big kahuna' which can cause owners a lot of expense and downtime till someone finds the small issue, or replaces a bunch of things and stumbles on the issue a good troubleshooter would have caught right away:
CORE BASICS:
A lot of people don't want to focus on the basics, but they're important and can be a the problem, or contributing problem. Clean your battery terminals and make sure the grounds to the engine/body are good (that costs nothing if you do it yourself). Seriously, the electrical signals go through the grounds so if they are so-so, that's going to affect signals to the computer, tranny, etc. Wiggle the fuses and relays to ensure there isn't any micro-corrosion affecting any circuits (takes 1 min, cost's nothing). Is the tranny fluid at the appropriate level? What is the condition of it? I might slide under the tk and wiggle the connectors at the tranny (again a 2-min job that costs nothing). Is the air filter dirty and plugged ..or new & clean? (can affect airflow if really dirty). Wiggle the MAF sensor connector. Are there any Check Engine lights on? If so, which? Can you hear any obvious vacuum leaks? All of these things above should take no more than 15 minutes to check, combined.
TEMPERATURE SENSOR:
Then I would probably start with making sure the computer is getting a good temperature signal (since this seems to be temp related). What is the temp the truck running at ....and is it accurate? Transmission shifting is controlled by the computer which gets it's information from sensors. And temperature sensors are a big part of that (so it pedal position, speed, airflow too). I would start with the temperature sensors (ambient and engine temp).
If you have a scanner, you could hook up and observe the temp when the truck is cold and compare to the actual outside air temperature. If you don't have a scanner, then you can take a multi-meter and ohm out the temp sensor pins at the sensor ..and compare to a temp/ohms scale. Sensors can drift. I had a bad one a couple different vehicles in the past and yeah, it screws up cold weather operation bc the truck 'thinks' it's different temp and delivering the wrong fuel, etc.
And do the same when the truck is warm ...but double-check the actual coolant temp by shooting the temp at the engine metal coolant outlet (where the upper radiator hose connects) ..with an infra-red temp gun. You want to make sure the actual temp is the same as what the sensor is reading when warm (the sensor the TIPM is seeing).
Beware some vehicles have two temperature sensors: one for the gauge, one for the computer. When that happens, the gauge can read normal, but the TIPM might be getting an erroneous signal. I honestly don't remember if the 5.7 in 2015 has one or two. I'm sure someone here will chime in.
MAF SENSOR:
From there you might ohm out the MAF signal. A faulty MAF, or even one which is at the low end of it's 'normal' range (but not enough to send a CEL) can cause problems too. What's it's voltage output? There are youtube videos how to check this. They can be cleaned with special MAF cleaner (results vary).
THROTTLE BLADE:
Oh, and you should also pull off the air intake to the throttle blade. Open the throttle blade and look for carbon deposits. I know it sounds unrelated, but, trust me, small things can make a big difference, and a lot of owners overlook this basic cleaning area. If there's visible dark varnish on the bore or plate, clean it with throttle-body cleaner (not carb cleaner) and a nylon brush (not steel or brass).
A shop makes money off a tranny replacement, not so much off the small things. That's where a lot of them will steer you. Don't overlook the basics.