Originally Posted by
B00STJUNKY
WK1? If so, I would suggest replacing the transfer case fluid. I just faced a similar issue with this in my 06 Jeep SRT8. I would get the clunk after start-up and it would occur around the same MPH. I would also get a rhythmic thudding/clunking in high gear and heavy load (full throttle going up a hill). I was beginning to think my transmission was on its way out, but I eventually tried disabling the T-Case by pulling the electric actuator motor, and after doing that, no more clunking. So, on a whim, and without much hope for it making any improvement, I changed the T-Case fluid, and everything suddenly felt fine.
Apparently these T-Cases (NV146) are super sensitive to having clean fluid in them. The fluid is expensive (to the tune of about $50/quart, and you'll need to buy 2 quarts), but it could prove to be the fix.