It sounds like you have a vacum leak the automatic hubs need vacum to operate. I would check out all your vacum hoses. My guess as to why that it wouldn't engauge is that when you are idling at a lower rpm after warm up the vacum has dropped off to a point where their isn't enough to engauge the system. You didn't say if you have a stick or auto but either way you could try holding the rpms at like 2,000 in neutral and see if it will go in then. If it does then more than likely its a vacum leak somewhere. If not then more info would be nice miles, last change of fluids, does it engauge while driving below 45, etc....