Transmission leak

It sounds like you have a front pump or torque converter seal that is leaking, it will be difficult to tell for sure unless you remove the transmission since both seals are inside the bellhousing. Here is a guide and video of how to remove the transmission to do the job with specific instructions in the diagrams below for your truck on how to do the job.

https://www.2carpros.com/articles/how-to-remove-an-automatic-transmission-rwd

and this video is a car, but the truck is very similar.

https://youtu.be/3vJAojsrfuU

This video will show you how to install the torque converter once the job is done, and you are ready to reinstall the transmission:

https://youtu.be/ZC7xHVMKLRw

Check out the diagrams (below). Let us know what happens and please upload pictures or videos of the problem.