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.
There is currently no way to reset item drops. The only way to start fresh would be to buy a new copy of the game on a new account.
If you are on steam the dlc is another purchased item so if you reset rl the dlc will still be playable afterwards.
Actually, if you didn't turn on steam sync, you could just delete the game and reinstall it and everything will be reset. I learned that the hard way, so make sure if you want to keep it to turn on steam sync.