Does anyone make a downpipe for the Dmax that has a vibration damper built into it?
The bellows are designed exactly like a vibration damper. They both allow flex of the pipe.
I believe the down pipe is bolted from the turbo to the rest of the exhaust system, not the manifold.
They are too stiff to expand just from exhaust pressure, I believe, and if for heat, why is there only one shown with the bellows on them whenever you see a set advertised?
It could be for all of the above. Trust me there is less vibration being transmitted from the exhaust manifold to the turbo, than there would be from the turbo to the rest of the exhaust system. The up pipes do not need to move since they are bolted from solid point (manifold), to solid point (turbo) but the down pipe is bolted from a sold point (turbo) to a moving point (main exhaust system) so engine vibrations would be transmitted through the down pipes. GM even had TSB's on this issue and not just for diesels.
I turned in a 2006 GMC 1500 (CA lemon law) for this very same issue that they could not resolve. That truck had other issues as well, but the vibration on it at 55 mph was incredibly annoying.