Bear, first off, you're right - it's W, not D. And thanks a lot for the info about other choices to convert.
The clunk happens on the road when I get on and off the fuel, and when I shift. If I do it calmly and softly, the slack is hardly noticeable, but it's there. When it first started, I pulled it over the pit and checked the driveline, my first thought being that I had a U-joint on its way out. The driveshaft and axle joints are all tight.
The background on this truck- It was owned by an old friend and run by him and others. Rode hard and put away wet. He had parked the thing, his health got worse and it sat in a shed for five years before he died. His heirs were going to scrap it and that's when I took it over. It needed surprisingly little to get it on the road. I did the front end - brakes, left axle, bearings and springs. That was five years ago. When I started it had 52k on it and it's got about 62 now. I run it around local in the good weather and my brother in law plows his driveway with it in the winter - very slowly and carefully. I can walk faster than he plows.
When I got it it had good rubber (ASR) rubber on it, but there were two distinctly different sets front and rear. I ran this for the first couple of years and then put a set of new lug tires on it, all the same size. I think that running the odd size tires probably accelerated the transfer case wear, but I didn't notice the clunk until afer I put the new rubber on it.
When I pull the unit into high lock (NOT on a dry hard road) the clunk either dissappears completely or becomes so faint I can't feel it. This is why I thought the slack might be in the differential, because if it was in the chain, you'd think it would be noticeable whether it was locked in or not.
I think the best and cheapest way out of this would be to convert the NP203 to part time. This truck is never going to do much more than it's doing now - no long hauls or long heavy tows - so for the price of the kit and a pair of Warn hubs (I think someone told me that the 9790s fit this) it could be made to work. I have a well equipped shop at my disposal - pit, overhead cranes, welders, press, all the tools - so all I need is the ambition and a word or two of advice from you guys. Or I guess I could just shift real smoothly, use an easy throttle foot and hope for the best. I've worked my way into my eighth decade, so the truck will probably outlast me whatever I do.
Thanks again for the quick comeback. This is a great forum.
Mark