2006 GMC Envoy Fuel Gauge Random Readings

I just bought a 06 envoy and have put about 800 miles on it when the fuel gauge went nuts on me. Sometimes it works and tells me correct fuel amount other times it sits and stays on empty. I use 97 octane or premium gasoline from the local Loaf' n Jug a kroger gas unit. Never had problems with the fuel before. I tried fuel cleaners, and injector cleaners. Finally went back to the dealership there was no warranty on fuel problems said $900 dollars would fix the problem the whole fuel pump needed replaced without looking at it. I have heard this is a sensor problem, but mechanic said the fuel pump for 06 envoy and sensor are 1 part and costs around 400 bucks. I can drop a tank and replace myself, but would like your mind on the problem since auto zone said there was a fuel pump and sensor seperate for sale on my envoy.

Thanks Chris