4X4 clunk, skip, loss of traction

[Heliguy]

Hi all, I have a 2005 FX4. Has 170,000km on it. Here is my problem. When I use 4X4 I get an ocasional, intermitent clunk or skip. The feeling is identical to driving over a large crack in the road or a small pothole or speedbump. The clunk comes in 2's, you feel it first in the front end then the back, like I said its just like driving over a crack. When it happens on icy roads the back end kicks out to the right and there is a noticable jerk in the steering wheel. It only happens at speeds in excess of 60 km/h or about 40mph. It happens on any road surface.. icy, snowy, gravel or dirt. Also makes no difference if I am driving straigt or around a corner but it will only happen when my foot is on the gas... not coasting. No dealer or shop can seem to offer anything helpful. I have done some reading on the net and from what I can tell this could be a transfer case problem??? Thanks for any and all help!