ABS/brake lights illuminated

My rear wheel sensor is located in the top of the differential and seems to fail frequently. I have replaced the sensor six times in as many months. I lose the ability to shift in and out of four wheel drive when lights are illuminated on dash. That is a large problem where I live! The only pattern have noticed is that when I drive in slushy conditions, my sensor will fail. I replaced the fluid in the differential last fall, although I have not taken the cover off to see if metal shavings are interfering. I will do this step next, although it does not explain the slush affecting the sensor. The only theory I have is somehow water is getting into a plug and is allowing more or less voltage to the sensor than is supposed to, maybe from pin to pin? Last fall I replaced the wire and plug to sensor, making sure to splice the new wire upstream of old splice. All of the old sensors test bad with a voltmeter as well. I do not have a wiring schematic of ABS system, and I am searching that as well. I figure I might be able to find plugs on ABS circuit that might be exposed to the elements and trace those back for testing. Any help is appreciated.