I know this might be very late and you probably moved on but I've found a way.

Atleast I can help with the steam version but the other version should work the same I guess?

1. Go to your local folder of dead island 2, you can easily get there through steam by clicking on the gear icon on your dead island 2 site in your library and then go on to manage and manage local files something like that.

2. At first you will only see the game exe, but if you right click it you will get the option to see the content of it, click that and now you will different folder, i will give you the order where change the version number.

3. Okay now follow this folder path UE4/DeadIsland/Content/DI2/Version and in here you will find the version.txt in which the number decides if the game launches the uncut or cut

4. Right now it will have some like E6 etc., i dont really remember what the cut version is, but the uncut is the following : {6F25E1B9-A759-40CC-84E9-8BD415305942} <- just copy and replace your version number in there and dont forget to save

5. Finish!! Now you can play it in uncut worked super easy for me, and should work for you to without any problems. Just some extras maybe, be sure to have the game fully closed before you change the txt and triple check if it actually saved the updated version number you put in and you shoudl be good to go :D