91 explorer 4x4 not engauging

I have a 1991 ford explorer and I'm having problems with the 4 wheel drive. When I push the button in it does nothing. If I push the button and hold it for a few seconds then the light comes on. But it still does not engauge. If I dont push the button again it grinds sometimes or makes a clunking noise until I back the vehicle up. It will then stop and resume normal operation. I got a pair of used hubs and put on it, but it still did the same thing. Does this sound like the shift motor?

Here's a link might be useful

http://www.automotiveforums.com/vbulletin/showthread.php?t=334898

Hope this helps sounds like the same problem.