2000 Ford Ranger Clunking Noise

I have a 3.0 6cyl. I had the switching motor replaced a few months ago as my trans switched (on its own) to 4wdLOW. Anyway, we just started getting snow and while driving the other day, I started feeling and hearing a "clunk" noise. It only happened while in 4wdHIGH and only happened when I was coasting (not accelorating). It seemed to be more severe when I was around the 40-50 mph area.
Any clues? It's been at the local mechanic since Friday and they seem to be stumped as they checked the trans, transaxle, fluids etc.