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I'm going to suggest that the vacuum system can be affected by temperature/moisture. I only have problems with my front diff trying to disengage itself while in 4HI when the temperature is -10 celcius or below and after extended highways speeds. It will start to humm rather loudly. If I throw it into two hi, and pull over, let the truck idle for a minute or so and then put it back into 4hi, I'm good to go again. I had the vacuum switch replaced as the last time this happened it chewed up my front diff. Still happens on occasion.
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