Ok I would first check all the vacuum lines to the front diff make sure none are leaking or cracked. If that is all fine then check the 4x4 module in the pass. side kick panal. Also may need to check the s4s shift motor. If you don't have a repair manual and need help walking through the module test then get back to me and I will look up that procedure. For now here is the vacuum diagram of the 4x4 system so you know what to check there.