KW T680 shaking problem

Having this problem for over a year now. It's driving me crazy. Replaced and repaired almost every single thing I could think of, but nothing seems to work. So... here's my problem.
Continuous oscillating shaking while driving above 60mph. When I have a light load shaking is worse. I'll try to describe it. It starts with light shaking for 10 seconds, after that shaking increases constantly for 30 seconds, after that, 10 seconds of smooth ride, and it starts all over again. The most intense shaking is located in the back of the cabin. Since that I'm driving in a team this shaking sometimes makes impossible for other person to sleep while in motion. Has anyone had this problem?

 

That was my thought too, changed the trailer and it is the same. When I'm bobtailing I can feel slightly less shaking but I still can feel it. It increases when I hook up any trailer. And as I've mentioned before it's worse when the trailer is empty

 

That was the first thing I did. Balance and alignment. Nothing has changed regarding shaking. Also replaced all drive tires two weeks ago... same

 

Shocks 6-8 months ago, also no difference after replacing those. I'm sure this is something so stupid and simple bc every obvious and reasonable solution is not working. I'll give it a shot with centering lugs... thank you