Funny that I can't seem to find the tach in my Bii electrical trouble shooting manual.... It does show a DG/Y wire going from the coil to the instrument cluster.....and spliced to go to the TFI, and also thru a resistor to the ECA.
Any way, the tach usually recieves voltage pulses from the ignition system, usually connected to the coil. a circuit in the tach converts the ignition pulse signals into a varying voltage which is applied to a voltmeter that indicates the engine speed.
That being said, I'd start checking connections between the two. I personally clean and use "bulb grease" on every individual connection that I inspect, just in case, even if it looks good. These trucks are 20 something years old. I like to pull every pin and socket out of the connectors, one at a time and clean them thouroghly, so I don't have to come back to it any time soon.
Don't forget the grounds. Make a new metal to metal contact (sand them with fine sand paper and use bulb grease, use a wire brush on screw threads), to insure future dependability.
Good luck, and go get'em!