I would say that Morrie seems to go on the physical journey as his health begins to decline due to his ALS: his body slowly deteriorates until he eventually dies as a result of the disease. Morrie was initially shocked at the news: Mitch writes how Morrie was "...stunned by the normalcy of the day around him. Shouldn't the world stop? Don't the know what has happened to me?" (8). As Morrie learns about his condition and of his impending death, though, he becomes more comfortable with the concept of dying.
Mitch seems to be the one on the "mental" journey. The book begins with him living life way too fast: "I had become too wrapped up in the siren song of my own life. I was busy.... I traded lots of dreams for a bigger paycheck, and I never even realized I was doing it" (33). Through his meetings with Mitch, Morrie begins to realize the important things in life, that money and fancy items do not bring true happiness. Morrie challenges Mitch by asking deep, penetrating questions, such as "Have you found someone to share your heart with? Are you giving to your community? Are you at peace with yourself? Are you trying to be as human as you can be?"
For an essay thesis statement, you could use "In Mitch Albom's book Tuesdays with Morrie, Morrie experiences significant physical changes in his lifestyle as a result of his ALS." For Mitch, you might say something like "Through his meetings with Morrie, Mitch Albom changes from being an egotistical, money-driven cynic to becoming a well-rounded, compassionate individual."