2002 Ford Explorer (4x4 Auto Question)

I just bought my 2002 Explorer a couple days ago (preowned of course) with approximately 42000 miles on it. They had to order me a new
owners manuel so I was unable to read up on this question myself... I have been driving my truck with the "4x4 auto" button turned on...figuring this means that if 4 wheel drive is needed it will come on automatically...however even with the recent snowfall here in Bucks County PA my 4x4 high light has been coming on even when driving on non snow covered/slippery roads. Sometimes it comes on when I am going around a turn or just accelerating while merging onto a highway...is this normal? I don't want to damage my drivetrain but it keeps coming on when it feels like..Any ideas on what is happening here? Does anyone elses Explorer do this?
Of course when I bought the car 3 days ago we had no snow so I just learned this was happening during the past 24 hours and since it has 42000 miles on it, it is no longer covered on any Ford warranty.....I am hoping it is nothing too serious.
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First thing that comes to mind is a question about the tires. Are they all the same diameter or are some worn significantly more than others? Others may have a better understanding of this, but I believe tire diameters are critical on the later model Explorers.
wrote:

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The tires are all brand new.
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