This sounds somewhat like <PERSON> Fionavar Tapestry:
The Fionavar Tapestry is a trilogy of fantasy novels by <PERSON>, first published in 1984-1986. The novels are partly set in our own contemporary world, but mostly in the fictional world of <PERSON>. It is the story of five University of Toronto senior law and medical students, who are drawn into the 'first world of the Tapestry' by the mage <PERSON>. Once there, each discovers his or her own role and destiny in the framework of an epic conflict.
To your specific points:
The basic premise was that there were many different worlds, but all of them - including our own - were just "shadows of the one true world."
In the books, Fionavar was the 'true world,' and ours was one of many different shadow worlds. IIRC, 'true world' and 'shadow worlds' are very close to the terms used in the books.
In the first scene, the protagonist (a middle-aged man) is visited by an old childhood friend. 72
The friend reveals that he is a wizard and asks the protagonist to come with him to the one true world.
This is exactly how the first book of the trilogy, The Summer Tree, opens -- <PERSON> has come with <PERSON> the dwarf to bring five chosen individuals from our world to Fionavar for a particular event. He reveals himself to five friends as being a wizard, and convinces them to come.
In a later scene, the two characters are riding in a truck, but when the wizard's spell takes effect, the scenery changes from a highway to a dirt road in the middle of a forest. Likewise, the truck transforms into a carriage.
This is where things diverge a bit. There is a scene in the series where some of the main characters are riding in a truck in our world, but I don't recall any morphing of the scene from highway to dirt road, nor a transforming truck-carriage.
Also, if I recall correctly, the friend was forced to go, leaving the protagonist alone in the city; other wizards were scheming to use the protagonist to further their own ends.
Circumstances did separate the five friends at various points throughout the plot, and other wizards were definitely involved in a lot of scheming, especially in the first book.
So.... not a perfect match to what you describe; but, close? 190