2000 Chevy Blazer Fuel level sensor in tank

I'm being told that my fuel level sensor within my gas tank is out. The gas pump is working and the gage on the dash works, just can't tell how much gas is in the tank (been driving it for the past 2 years with no problem except for NOT knowing how much gas is in the tank). When I fill the tank the dash gage will show full and as I use the gas it will move to the 3/4 full line on the gage then stop. The low fuel light will come on when the tank is almost empty. Sometimes the gage needle will bounce back and forth from empty to full. I was told that I will need to replace the pump and sensor within the tank because it is all one unit.
Is this true? What part do I need to get to fix the problem?

Thanks for your help!