Non-localized vibrations at decreasing rotational speed

Hello,

While I drive a Fiat Stilo JTD, I think some people might have some ideas.
Engine: 1.9 JTD, 115 horsepower, Common Rail. Mileage: 58,000 km.

"For about 5000 km, I've been experiencing noticeable vibrations when I release the accelerator pedal at engine speeds above 3000 RPM and allow the car to roll in gear. Initially, this only occurred in the lower gears, but now it's happening in all gears."

The mechanics are stumped. Otherwise, the engine is running perfectly. I'm almost suspecting the transmission or the driveshaft. But those are things that you can't just easily replace or test.

Maybe you have some ideas?