First step is to consider the more likely wear of universal joints on the propshaft.
Get the car up as high as poss (jack with axle stands or on a hoist) and grasp the propshaft firmly and twist backwards and forwards. There should be no discernible movement. If there is it's relatively cheap to replace them. Subsequently you may find there is still a clonk but the UJ's would have needed replacement in any case. If it persists then take it to a gearbox specialist to see if there's any play in the GB, otherwise your then left with the axle as the culprit.
Usually it's the UJ's
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