High Temp Gage on 2001 Explorer

I have a 2001 Ford Explorer Sport Trac that's running hot. The needle on the temp gage runs to the top in a manner of minutes while driving. I just had the coolant changed, but it didnt help. Any ideas on what's wrong? Is it the water pump, or the sensor, or maybe the transmission? Should I go to the dealer for service or an independent shop for repairs? Thank you. -HJI