We don't get to know Tracy very well in This Is Where I Leave You. For the most part, she's only there to illustrate how Phillip is trying and failing to become a functioning adult. There's really only one scene that we get insight into Tracy as a person—her conversation with Judd on the roof, where we see plenty of similarities between Judd and Tracy's respective struggles.
Like Judd, Tracy is entering the middle of her life uncertain of her place in the world. Of course, she seems well put-together. But would a stable woman really date someone like Phillip?
While Phillip looks to Tracy to provide stability, Tracy looks to Phillip to connect her to his carefree lifestyle, sort of the Manic Pixie Dream Girl with the gender reversed. (Check out our discussion of Penny for all the inside information on the MPDG.) She knows it, too—she tells Judd that whatever lies he told her "pale in comparison" to the ones she tells herself. (43.21).
Is that really much different than what Judd is going through? They're handling their midlife crises in different ways, but their struggles are the same. In fact, you could argue that she inspires Judd to take control of his own life more than anyone else in the family.