I want to reset my stuff on rocket league so I can unlock everything again for fun, but I don't want to lose my DLC items. Is this possible, and if so, how do I do it?
Yes, according to this reddit thread, both your ranks and items are stored locally.
Your save file should be located in a folder inside
\Documents\My Games\Rocket League\TAGame\SaveData\. Deleting it will reset your progress.
However, if you have Steam cloud synchronization on, you will just download the save from the cloud, so you either need to turn the cloud synchronization off or forcefully overwrite the cloud contents (see here for an extensive description on how to do so).
Your DLC items should still be available (but make a backup before trying), since it acts the same as if you bought the game with all DLC and started it for the first time.