Oh oh. Let's not go there again. Do a search, there was a long thread on this subject last year. I think you'll find many opinions and reach no conclusion.

 

'Passenger seat shake'.
This along with the rear main seal issue, are 2 of the biggest Porsche problems.
Of course.....I am kidding .
If the car's suspension is 'relaying to you' something that's on the surface of the road....that's one thing.

If you find a nice smooth new stretch of asphalt, and you still get the same feeling, then the car must be 'manufacturing' what you feel (not transmitting it).
That's when you start questioning tires, inflation, balance, alignment, etc. and not whether Zuffenhausen designed it that way.
I have non-standard suspension (read 'stiff') and I often warn riders before we take off....but also, I would not expect any Porsche or high end car to 'shake' anywhere between 50 and 150 (or more).
Some generalizations that may or may not help.
*If the feeling is speed-sensitive (you say above 70, but not at 50) then balance is likely......you can 'prove' that by going 85 and it won't be as intence.
*Are the tires quiet?....or are they making some noise as you decelerate. Poorly aligned tires can easily 'cup' in 4K.....once a wear pattern is established it only progresses.
*Cheapest thing you can do is have a good shop spin up all 4 and correct what they find and let you know what they find.....should be no more than $30/wheel.
*Balance....select a good shop (or the dealer)....always let them know you will expect a print showing all before & after settings (ploted against OE specs)......keep the sheet as a reference for your next visit.

As others have suggested....if it doesn't feel right to you, then take time and figure it out. These are not boulevard cruisers but even a rough GT3RS doesn't 'manufacture' shake at speed.
Good luck sorting it out.