Battery problems of the 2004 Chevrolet Avalanche

I have a problem with the speedometer showing inaccurate speed on my 2004 chevy Avalanche. The problem started while I was slowing from 55mph to 45 mph do to a reduce speed area, the vehicle slowed but the speedometer said at 55mph. It stayed at 55mph even when I came to a complete stop, shut off the vehicle and restarted, when I put the vehicle in gear and started moving again the speedometer began to move up to 60mph when I was traveling approximately 10mph. I sped up a little more and the speedometer climbed more. I slowed down to a stop and this time the speedometer stopped at 60mph while I was at a complete stop. I turned around to go back home as I could not legally judge my speed and as I proceed to travel at my best estimate 50-55 mph the speedometer climbed up past 120mph. When I parked the vehicle at home the speedometer showed 65 mph. I opened the hood, disconnected the battery and let it sit unconnected for approximately hour and then reconnected the battery. This did not help, the speedometer still showed 65mph. I contacted my local chevy dealer and asked them about the problem I was having, they told me that since I have over 40,000 miles on my vehicle that I would have to pay 400 dollars for a new cluster. This is a safety issue and it should be covered under warranty. I can not drive my vehicle for fear of inadvertently speeding and thus getting one of those nice driver awards for speeding. Please help resolve this matter.