Its probably for a kickdown cable... though I don't think these trucks ever had one.

"Throttle kickdown: Most automatic transmissions include some means of forcing a downshift into the lowest possible gear if the throttle pedal is fully depressed. In many older designs, kickdown is accomplished by mechanically actuating a valve inside the transmission. Most modern designs use a solenoid-operated valve that is triggered by a switch on the throttle linkage or by the engine control unit (ECM) in response to an abrupt increase in engine power. The kickdown feature is designed to never select a gear ratio that would result in engine overspeed."

http://en.wikipedia.org/wiki/Automatic_transmission