1999Ranger XLT 4wd Hubs

I have a 1999 Ford ranger XLT 4wd. The front hubs don't seem to be engaging.I hear a noise like something is moving and the light comes on the dash but the hubs don't engage. How do these hubs work? do they work on vacuum? I would like to replace them but Wern dosn't make them for the 99. Thanks