MY GAS GAGE ON MY 1999 CHEVY VENTURE DOESN'T WORK. IT ALWAYS SAYS FULL. DO I NEED TO REPLACE THE WHOLE GAS TANK?

NO. 1) I would check to see if the gauge is faulty, 2) check to see if the wiring is faulty, 3) drop the tank and check the sender unit. It could be faulty or it could be stuck because of trash or gum accumilation from old gas. By the way if it were me and I had the tank off I would go ahead and replace the fuel pump while it's easy.

NO, on the fuel pump assembly is a swinging type arm with a float on the bottom end and some metal teeth on the other, as the fuel leval goes up or down these teeth contact a plate on the body of the fuel pump assemby that send a signal to the gauge. Now either your float arm is sticking, guage is bad, or the sending unit is bad. I would suggest the sending unit, it is availble as a repair unit, you dont have to buy the whole fuel pump. The gauges usually last the life of the car.