Shifting issues

[Capitaldistrictproperty]

Bought a brand new 2019 ram 3500 crew cab duly it does have a 6.7 L Cummins in it with the 68RFE transmission truck has a little over 36,000 miles on it I have noticed this problem since the truck had around 10,000 miles if I really accelerate hard up a hill while pulling a trailer it will try to find a gear from 5th to 6th but it bounces back-and-forth cant figure out what is causing this and every now and then it will kick the check engine light on and the code of fifth gear incorrect ratio will appear the truck is deleted and now I have never ran anyProgrammable tones in it just the stock tune the reason the truck was deleted was because a DEF line went bad and it was back ordered for over six months in my truck kept going into limp mode The problem is I cannot take it to the dealer because they will automatically blame the delete tune on it and void my warranty even though the trucks sat in their lot for three months while waiting on a part and finally I got frustrated went and had it towed home and deleted it just so I could take it to work so I’m kind of on my own any thoughts or opinions would be great