Jimmy 4WD problem HELP

Hi, my 97 Jimmy wont go into 4wd. This just started happening. The electric dash board switch works fine and you can here and feel the transfer case swith, as well as hear and feel the front drive shaft engage. But no 4wd??
I checked the vacuum actuator under the battery, it pulls in when vacuum is applied. I remoced the cable from the vacuum module and pulled it with pliers thinking perhaps it broke and might pull out of the casing but it didn't.
I really am lost here. A GM dealer suggested something inside my front axle was broken and to bring it in. I'd rather ask here first before investing hundreds on what might be a simple solution. Thanks

Check vac lines and make such the front act is pulling the coupler all the way in or that sensor will not trigger telling the TCCM good to go into 4HI or 4Lo. That connector on the front axle is a ball type sensor so check that - when coupler engaged the ball pushes up - you can take it out and test it. Then see the many posts on many other fixes for the 3 button np233 case.