Very hard shudder/shake when accelerating in 5th gear

[nr16]

Hey.
I’m having very disturbing issue with my 2003 Volvo XC90 2.5T.

When cruising and overtaking at city speeds all is ok.
When trying to overtake someone on highway (100km/h / 60mph), pressing accelerator makes the whole car shake as if axle splines would try to make contact with transmission or wheel bearing. The shaking goes for 1-1.5secs and then the car downshifts to 4th or 3rd depending on how far I pressed pedal and starts to build up speed.
While shaking, revs dont change but stays the same.

1- when i downshift manually from 5th to 4th in MAN mode, all is ok, there is no shaking. The shake is only when in automatic shift mode.
2 - if accelerating while cruising at 120-130 km/h there is no shaking, it downshifts really well by itself
3 - have drained the gearbox for 2 times now this month, doesnt seem to have milky texture if coolant would be present in atf. Will do drain and fill 3 more times to make gearbox cleaner. Shifting is really smooth and good for car this age except that one bit on highway
4 - when cruising in 5th gear around 2000rpms there is this low pitched vibration. If shifted to 4th gear, revs go a bit up and vibrating sound eliminates. I don’t wanna drive in 4th because of fuel economy. What that could be? I changed both rear wheel bearings a month ago. No change.

has anyone had this shifting issue?
what could it be?

front right driveshaft bearing?
propshaft center holder bearing?
sparkplugs?
throttle body?