Bart,
The drag link has a ball and socket pivot on each end (I think). I don't recall those making a clunk when the suspension flexes as you describe, but I suppose that could happen. Usually loosness in the drag link causes looseness in the steering, noticable by slack in the steering wheel. You can also see movement in the socket(s) if someone moves the steering wheel left and right a little while you watch. Drag links have a large slotted plug that can be adjusted to tighten the joint if necessary, and the parts are not worn excessively. After tightening the plug, you secure it with a cotter pin.
Harvester
