If it's private, not for hire, then no, you don't need a special license.
If it's private, and for hire, and over 26,001gvwr, then you will need at the very least a class B.
If you are a firefighter driving a vehicle registered to a fire dept based in Washington State, and you have taken the appropriate EVAP course, then you don't need a CDL either:
http://www.dol.wa.gov/driverslicense/cdlexempt.htmlFirefighters and law enforcement personnel are exempt when operating emergency equipment if they carry the certification card proving they have completed the Emergency Vehicle Accident Prevention Program (EVAP).Washington State is pretty clear on it....some states say "when operating emergency vehicles on emergency calls" therefore leaving a huge gray area for parades, events, taking the truck in for service, etc.