1995 Dodge Dakota Gas Problem

Engine Performance problem
1995 Dodge Dakota 6 cyl Four Wheel Drive Automatic

Almost two years ago my truck begin to slow down to about twenty miles an hour when it got below about a half tank of gas. The RPM is extremely low and the pedal can be held to the floor. It continues to drive but it does not reach a drivable speed (espcially on highways). If I put just a gallon of gas in it, it drives fine. But once it reaches that halfway mark it slows down again. It doesn't seem like it would be a fuel filter problem because it sits in the bottom of the fuel tank so it is always surrounded by gas (since it is still half full). The fuel pump still works after two years so I don't think that is the problem either. Please help.