As SB states, a a poor connection at that sensor will default the high fan to RUN.....
That sensor on that chassis it the blue, 2 wire one at the thermostat housing.
You can also test the circuit by unplugging that connector and bridging the 2 wires with a 1K ohm resistor...that will put the fan off if the circuit is OK..if not, then the suspect is a sticking high fan relay..that is under the panel [ 6 screws] behind the fuse box..the Green relay..

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A Dalton