AWD Transmission: Heavy vibration under load

[Darkside]

Hi,
Hoping for some advice:
2005 XC70 AWD Auto ~220,000 miles
When under heavy load, towing a caravan, accelerating in 5th gear (from 50 ish MPH) there is massive vibration through the whole car.
If I either manually shift to 4th or kick down to 4th it seems to pull ok with no vibration.
My dread is that there's a problem with the torque converter or the DMF, both of which will make the car uneconomical to repair due to age and mileage and looking at the value in the car.

I've done several searches over the last few days, and most results point to front axle(s) but I'n not convinced this is the problem here.

Any advice would be greatly appreciated.

Chris

[Georgeandkira]

Worn (thin) and "use polished" surfaces in the clutch? Pure guess on my part.

[Darkside]

Hi,
It's an auto box.
Cheers
Chris