There is a soleniod set on the valve body that controls pressure and shifting they go bad occasionally. If it shifts good leave it alone.

 

I think how you use your truck makes a big difference on service intervals. If you tow or haul heavy a lot I would service tranny more often than if it's just a daily driver.

 

Waste of time and money ( you need more than a drain plug to exchange all the fluid in the TC any way)....just change the pan fluid and filter every 20,000 miles. Ask ATS what fluid they use and recommend and use the same.