Welcome to the Forum from NH.
Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.
I hope that you will become a participating member in the Forum's discussions.
Pics of the truck, please.
You are already receiving sage advice from the knowledgeable folks on this Forum.
If you don't have a quality bi-directional scanner, you should consider purchasing one. It will literally save you thousands of dollars over the life of the truck, and it will most likely pay for itself in less than 1 year.
I case that this ends up being a transmission issue,
@NickTransmissions is our resident transmission expert and he may provide some insights.