Check for vacuum leaks first, then there is a vacuum switch on the transfer case, or the vacuum can on the front axle is broken. I installed a Posi-lok on mine. There is a ,model with a cable to actuate the front axle, or one that keeps the CAD locked all the time. I have the one that keeps it locked, I don't put a lot of miles on and I figured the axles are splined to the hubs and spinning the gears in the axle all the time anyway.