97 S10 Blazer 4x4 Problem

97 S10 4x4 Blazer, 4.3 V6 (not ZR, just std). Front differential not engaging. I jacked up all four wheels, push the 4H button and I hear a clunk and the drive shaft going to the front differential engages but front wheels don't turn. I unhooked the three bolts that hold the acuator cable in the differential but didn't unhook cable. I pulled on the plate/cable and the front differential engaged, front wheels turned, no noise. I checked the vaccum acuator with cable disconnected and it moved thru the full range of motion. I unhooked the cable from the differential and when 4H is pushed the acuator pulls the cable thru full range of motion. No apparent vaccum leaks, but when I hook everything back up and push the 4H button, the front wheels don't turn. There was a clicking sound coming from front differential when like it was trying to engage but couldn't.

Any help would be appreciated.

Update: I installed a new vaccum actuator, flushed and changed the differential oil, still only way I can get front differential to engage is to assist the actuator by squeezing in on it with my fingers. I'm thinking it must be low vaccum. Where does the vaccum come from? The hose runs to the back of the engine then it's hard to trace, looked like it was going toward back of the vehicle, but I need to follow it all the way back to source of vaccum.

Is there a vacuum "ball" mounted under your hood, about 4" dia. on the drivers side?