Most likely the sending unit. To my knowledge, they are not repairable.
To replace it, I suggest lifting the bed off, it is much easier then dropping the tank. Depending on the modifications to your truck, future modifications, and lift pump type, I suggest this as a replacement:
The same thing recently happened to mine. My first thought was the lift pump went (Truck died at 65mph the day after i bought it). Had an Airdog 100 installed (no draw straw) Died again at 1/4 tank. Put 7-8 gallons in it, purged lines, went to get fuel, it still took 25 gallons. it was completely empty at 1/4 tank on gauge. I plan on ordering The Vulcan module ASAP. Until then i will be filling up at half a tank.
BTW- Thanks for your help on that RAW