When I change the differential fluid, what fluid should I use?

They make a many fluids for your truck. I would go with a synthetic from Mobil1 I use it in all of my vehicles. 90w. This video shows the job being done on a similar car but the process is the same.

https://youtu.be/hpcxcnKbVto

Here is a guide to help you step by step with instructions in the diagrams below to show you how on your car.

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

Please run down these guides and report back.