Front flex disk is the most likely cause, but you also need to check the pinion for excessive "lash" -- usually not the ring and pinion gear, but instead the carrier for the "spider" gears in the differential. The bearing for the shaft wears oval, and it makes a terrible clank.

Take a look -- if the flex disk has cracks in it, replace it. If the driveshaft turns a half turn or so and makes the clunk in the diff, you need to have it checked, might be time for a replacement.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!