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'99 chevy s10 zr2 4x4 squealing

I have a 99 s10 zr2 when I have driving down the highway going 60 to 75 mph it will start making a squealing noise. The more I slow down the louder the noise gets. If I push the 4x4 button The noise stops then I put it back into 2 wheel drive and its good to go for a while but will happen again I think it might be the vacuum actuator under the battery but not sure. Any help would be much appreciated