Quote:

I can sometimes actually hear the body structure 'work' at the rear because of the rearaxle and leafsprings putting more and less strain on the suspension-mountings.



Pretty sure if you are hearing things your problem is elsewhere. I'd go over the rear suspension and make sure nothing is sloppy, wore out, or rubbing at extreme angles.

I'd work on chassis improvement front to rear, what happens if the rear is stiffer than the front?

Bad things.

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Don't be the exception.