Good score! Does it have the 4.3, 5.0 or 5.7? Son of Weldangrind has a '98 with a 4.3 that looks just like yours. His has a five speed.
The heater motor resistor pack has failed. When that happens, the default is no resistance, which means high speed. This might be the one:
http://www.napaonline.com/Catalog/Ca...R76_0239905447