Q: Why do i have to put it in reverse before 4X4 kicks in. on 1991 Chevrolet S10 Blazer

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I have to back up before my 4x4 engages/
(1) Answer
You shouldn't have to. You should be able to engage and disengage 4 High while you are driving. There was an engineering change kit that was supplied to the dealers that may have something to do with it. It has a vacuum sensor and a wiring harness that has something to do with engaging the front axle, and a replacement vacuum switch that goes on the transfer case. Check with a Chevy dealer to see if it needs to be installed on your vehicle. If so, they will do it for free, even at this late date.