Hello PGorman and welcome to Just answer.com, Super mechanic here. About your truck.
The fuse in the panel for the tail lights I am pretty sure will be the #13 fuse. But it is always good to check them all. There should be power on both sides of the fuses.
There is a fuse box that is under the hood.
I am pretty sure the fuse for the tail lights will be marked. You might want to also check the headlight switch and make sure there is power coming out of it when the lights are turned on.
I have a diagram that is similar to what yours is. It might give you an idea of how the system is powered.
You might try the ground that is under the bed. I am pretty sure it is on the left side, there should be 2 or 3 wires that go to a ground connection at the side of the frame.
Also, wiggle the harness for the rear lighting, (in the back). There is a connector there that might have an intermitant problem.