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Pump shouldn't be running with ignition off. Should only run when in 'run' position.
Sounds to me like you have a bad ignition switch, stuck relay or somethin is back feeding.
Very similar to my issue I have which I think I have solved now. My fuse panel under the dash had got water in it, as had the GEneral module. I had similar symptoms. Erratic speed sense (tried all the usual changing VSS etc) and an electric hum under hood. Wasn't vacuum pump though.
Sounds like 2 issues. Ignition circuit staying energized after key off. And a vacuum leak. Since pump should only run until reaches set vacuum pressure. Maybe pump, or reservoir cracked. 4wd pulse solenoid leaking or damaged line somewhere.
Worth changing speed sensor, since not that expensive.
But I'd see if you can see any sign of water or corrosion under dash panel behind fuses.
Try unplugging the pump and see if your batteries drain again.
7.3l DRW F350 4x4. stock, with exception of Airbags on rear, bilstein all around and 26k 5th wheel hitch. K&N filter for good measure.
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dont try this at home kids!