[kaiser]

It could well be from the gearbox.
The torque converter lock up works in Drive. It is like a couple of hundred revs increase when you put your foot down a bit, or hit a hill, as the car goes out of lock and into normal converter mode. If that is not enough, it will change into 4th.
If you drive in Sport mode, it will never switch the lock up in, and you will not get that rev increase from the torque converter.
So doing the judder in Drive and trying to do the same in Sport, and you will see if it is the gear change itself or maybe the lock up.
Apart from that, I can only advice you to do an oil change on your box. It might cure it, it might not, but it is relatively cheap and most likely needed.