Are Millennialism and Tribulationalism the same?

Very simply put: No. The best way to separate these segments of a larger study, eschatology, is to start with the former and focus down to the later.
First, defining the term eschatology is important. This is the study of “End Times”, or “Final Things”. Within this specialty are many topics of interest. But it seems that most people who are starting out in their study of the “End Times” confuse these two.
If a person can get a foundational understanding on how these two are different, or what role or part they play with each other, then one can begin to build this part of their belief system within this framework. Once this framework is established, and then studying the scriptures will allow a person to place each new block of information into their framework. This will allow a person to begin to develop a cogent understanding of what the Bible truly says about the “Second Coming of Jesus Christ”.
The term millennium comes from the Latin word for “thousand years.” This is the thousand year reign of Christ mentioned in the book of Revelation. Premillennialism is the teaching that Christ will return to earth and establish His kingdom and reign for a thousand years. Postmillennialism is the teaching that Christ will return at the end of the millennium. In other words, the world will get better until the world turns into a millennial utopia. Christ will then return. Amillennialism is the teaching of a figurative kingdom. The initial “a” is a negative particle. This means “no millennium. Adherents believe that there will not be another earthly manifestation of Christ’s kingdom. The reference in the book of Revelation is figurative. Christ’s kingdom is in the hearts of believers and in heaven.
Now, let’s look at the subject of “Tribulation” or the “Great Tribulation”. This is the seven-year period, which will precede the millennial reign of Jesus Christ. There are three positions about this period. So, when we talk of the “Great Tribulation”, we are talking about a position that is premillennial. So all of the tribulation positions are premillennial. What differentiates the three positions is when Christ is to return to earth. These positions are the pre-tribulation, mid-tribulation, and post-tribulation rapture of the Church. The first position believes that the Church will be raptured out of this world as the tribulation begins. Mid-tribulationists believe that the Church will be taken up at the midpoint of the tribulation, the 3½-year point. The latter position teaches that the Church will go through the tribulation. Christ will return to the earth at the end of the tribulation, receive His Church, and establish His kingdom.
So, you see that Millennialism and Tribulationalism are two different subjects. The latter being part of one of the millennial positions, premillenial.