My '81 300TD Turbo does this too, but not that badly. It seemed to get better after changing the intermediate (center driveshaft) bearing, mount and transmission mount, but it still isn't totally gone.
I start getting some vibration at around 75mph and then it smooths out at around 80 and nothing really noticable beyond this.
Any rotating part will have some vibration, and they all have a resonant range where the vibration 'feeds' itself and it becomes beyond what would be normal, and in some cases beyond normal enough that they start to self-destruct. I don't know what that range would be for any given part and I don't know if anyone ever really cared to figured it out. But, I'm sure that whatever it is you would never be in the range if everything was within spec. Things out of balance could obviously change where this range begins and ends.
I would think gear related things would whine more than anything else, unless something was coming loose. Driveshafts, especially the rear half, could easily be the culprit if out of balance. I know the balance weights tend to rust and get thinner as time goes on, so it wouldn't be too far fetched to think this could be a contributing factor.