4x4 won't engage

i have an 89 cherokee and the 4x4 won't engage. I hoope its the vaccum but i dunno how to check to be sure...any little help will be greatly appreciated!!

Check for vacuum at the switch on the back of the T-case. First, can you engage it at the T-case, just not at the front axle?

It engages at the t-case. I just don't think its gettin Enough sunction from the vaccum.

It takes small amount of vacuum to engage the front axle, barely noticable. I would first remove the switch from the T-case and make sure its not stuck. Mine was. If its good, double check your lines ,then remove the vacuum motor assembly from the axle and inspect the diaphram. <ost times though it'll be the vacuum switch on the T-case.