Common complaint among new Tig owners. The transmission shifting is programmed for maximum mpg, so downshifting occurs later than normal (i.e. at a higher speed). The "trembling" point seems to be about 35 mph.
Now the "good" news is that allegedly the Tig "adapts" to your driving habits and so you might see an improvement after a few hundred miles. Now sure how this "adaptation" works.
I have 1300 miles on my Tig. To be honest, I hadn't really noticed a problem until I read about it online (about 700 miles ago). It seems to have gotten better (less trembling at 35 mph, sooner downshift by a few mph), but I wouldn't bet a body part on it, it could just be wishful thinking.
Interesting you should notice it. I read that it was only a North American issue.