When running in 5th and 6th gears I lose a lot of power-usually 45mph is top end speed at times? Have all the power needed in low end gears but nothing in high end. Any thoughts or ideas would be helpful.

 

I would start cheap and change the fuel filter first. If its the '96 you have listed at the top then it could also be the overflow valve.

 

I am on the third fuel filter in as many weeks. I have also changed the outer air filter, havent found a parts store that carries the inner one yet.
I recently added another fuel tank to the passengers side of the truck, it is only gravity fed to the main fuel tank. Could this be an issue?