I'd say the difference between a shitpost and trolling is malice. Trolling is done to get a rise out of someone while shitposting is meant to be treated as just detached absurdist humor.

With many people on social media being increasingly touchy about the slightest perceived offenses, calling something a shitpost acts kind of like how /s indicates sarcasm. IRL it's easy to tell when someone you know is joking when they say something that might be wrong or offensive, but the same mechanism doesn't necessarily exist online, thus the rise of "shitposting" as a term.

E.g. if I were to say something in favor of flat Earth theory, people who know me would instantly know I'm being absurd for the sake of humor but someone seeing the same posted online might assume I'm a flat Earther.