funny gas guage

Hi folks. The gas guage in my '83 diesel P'up is wierd. When it gets down to about 1/4 tank, it drops off. I mean it almost instantly goes to well below
empty. One minutes, I have a measurement, and a block or couple minutes later, nothing. I don't think the guage is fualty as it does work(about 1/4 to full anyways). The wires under the dash seem ok with no shorts or loose wires. Could this be a problem with the sending unit in the tank? It's frustrating with my fuel running $2.19 a gallon here.
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