I'm having gearbox problems in my 2013 Ford Territory

The first thing to do is get your mechanic to drive the vehicle when it’s hot. Arrange a time and then deliver the car at that time with everything up to full operating temperature. Otherwise, the problem isn’t going to show up when it needs to.

While there are lots and lots of things that can cause this sort of behaviour, it does sound like something in the actual powertrain is the problem. That’s because the shuddering goes away when you drop the transmission into neutral and let the vehicle coast to a stop. If the problem was a wheel, a wheel bearing, axle, brake problem or an out-of-balance driveshaft, the problem would continue even though you were coasting.

You may have a problem with something inside the transmission itself, way too much backlash somewhere in the driveline or a problem with the torque converter. Either way, you need to have your mechanic experience the problem or everybody is simply guessing.