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I am not a 4wd expert, but I would assume it's similar to other 4wd I've driven.
When you first shift into 4wd, the transfer case engages your front drive shaft. The "clunk" you hear is this engagement happening (normal) as the shafts slide into alignment. If you shift back into 2wd, the tranfer case disengages, but the free-wheeling hubs on the front wheels keep the front drive lines turning (but not powered by the transfer case). The driveshaft is simply spinning along to keep it "lined up" or "meshed" with the transfer case. When you go back into 4wd, the transition is smoother, because all the gears/splines on the shafts are still in line.
Once you disengage the front hubs (usually by reversing in 2wd), the front drive lines no longer spin, and come out of alignment with the transfer case. Inorder to reengage the transfer case, the shafts again need to re-align - - thus the "clunk" again.
So that's my explanation. I'm sure there are guys here that can explain it better, but that's the jist - - at least how I understand it.
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Zilverado
1992 Pathfinder V6-SE 4wd
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