Low oil pressure signal is beeping in the truck?

Yep, this happens when the oil pump pickup gets clogged due to sludge or the oil pump pressure relief springs breaks. How does the inside of the engine look is it sludged up? If so, I would start by removing her oil pan to clean the oil pump pickup which is most common.

This guide can help as well:

https://www.2carpros.com/articles/high-or-low-engine-oil-pressure

Here is how to remove the oil pan in the images below. Check out the images (below). Let us know what happens and please upload pictures or videos of the problem so we can see what's going on.

Sometimes you can remove the oil fill cap and look inside. If the inside is clean then I would suspect the oil pump being bad or an internal oil galley plug has fallen out. The only other thing I can think of is to install a manual gauge to make sure the oil pressure reading is accurate.