It is very possible that your driveshaft u-joints are either frozen, or bad altogether. Mine was clunking similar to what you describe, and when I changed the driveshaft center bearing, I soaked the u joint with rust penetrant, and then a lot of penetrating oil/grease and freed it up. Now, no more clunks and muuuch smoother on the highway. Just my .02

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)