I would like to know how to do a gear oil change service please?

Hello, I'm Danny.

Here is the information you requested. The service manual shows that the rear differential gear oil is recommended to be changed at 150,000 miles. Here is a tutorial showing how to change differential gear oil:

https://www.2carpros.com/articles/differential-service

I've attached picture steps below on how to change the differential gear oil including the recommended type on your truck. Hope this helps an thanks for using 2CarPros.