Well the tank is probably full if even the 5-gallon tank cant fill it anymore. This means either the sending unit was damaged when you switched the tanks or it simply isn't compatible with the new (used) tank.
Here's a way to test it. Either drop the tank to get to the sending unit or take out the gas gauge and get to the wires that come from the sending unit. One will be negative or ground and one will be positive and will go right into the sending unit.
Use a voltmeter to measure the Ohms of resistance between the wires that run to the sending unit and ground. With float in up position, resistance should be 20 ohms. With float in down position, resistance should be 220 ohm. If it reads close to 220 ohms but the gauge only reads half, you probably have a problem with the gauge. If it reads something like 100ohms, then the gauge is probable fine and its probably something wrong with the sending unit or the tank itself.
As far as the ping pong ball, I can only speak for chevy sending units, but I assume they are all similar. In my old chevy truck, their are 2 lines on the top of the sending unit. One that feeds the engine, and one thats a vent. The little pingpong ball is on the line for the vent so if gas splashes up (like of you are going up a steep hill), the little ball will plug the line and stop gas from going out the vent. When you are level again, the ball should drop and allow gasses to enter or escape. It is possible you are having "Vapor Lock" in the tank because the vent could be plugged. Again, I havent seen a TJ fuel sending unit before but it should be a similar idea.