2017 Denali 3500 Duramax-driveshaft vibration

[andypa]

I just got my brand new Duramax Denali 3500 dually a month ago and when I hit 65-70 mph there is a noticeable vibration that feels like the rear wheels or driveshaft. It only occurs when in 6th gear(Alison) and light throttle. If I shift into 5th at the exact same speed there is no vibration and if I put my foot into it ( gas paddle ) there is no/minimal vibration. I can go 80 or 90 mph and it is silky smooth.
I had the wheels/tires roadforce balanced and it made no difference at all. The picoscope showed lots of T1 after the road force balancing. More than before, but before there was also some E1 readings whereas those dissappeared after the roadforce balance.
At this point the dealer is at a loss and they did contact GM technical who have only let them do the roadforce so far.
Anyone have any ideas. This is the most expensive vehicle I have ever owned and it rides like shit as most of the time is spent around 70mph.

 

[98trck]

hey I'm just thinking out load,
it's a dually, can you remove the outer wheels and still bolt the inner wheels tight?
I believe you can...
how about trying that and see if you have the vibration.
PETE

 

[andypa]

Not a bad idea. I might try that tomorrow. Had it in with another dealer as apparently their RFbalancer is broken. Said they kept getting different readings each time they put it on the machine. Nearest one to me now is 300 miles away, booked in there on the 27th.