Josh, you will have to expand on this in class, for I don’t know that I agree (perhaps I simply don’t understand). Dealing with themes of loss of innocence and alienation (I assume you are linking the work to T.S. Elliot-esque imagery) isn’t necessarily a hallmark of Modernism or Postmodernism. The telling feature may be how the characters come out on the other end. (i.e. Holden Caulfield laments his inability to erase the word “Fuck” or be a “catcher” in the rye. Yet, he lets his sister reach for the brass ring at the end without trying to save her. Eliot’s Prufrock doesn’t dare to eat the peach. In other words, the characters have changed as a result of their alienation.)
The real test here is whether or not Oedipa has been affected by her hallucinatory journeys. She may not have solved the mystery, but (to borrow a film metaphor) has her character “arced” at all? Some critics claim that she has evolved as a result of her journey (i.e. the false pregnancy, her self reliance.) This, more than the themes (for I’m sure we both can think of a dozen PoMo characters that are alienated and disenchanted), would align Pynchon with the modernists.
If I misunderstood your post, a thousand apologies.