Hi Elisa, 

We use "kidding" informally and casually when we are teasing someone and looking at their response. 

Example: Hey, you forgot to zip your pants! Haha... Just kidding!

We " crack a joke" when we are more intentional on making others laugh by presenting something hilarious. 

"Joke" can also be used to indicate our disbelief.

Example: You must be joking! / You can run at the speed of sound? What a joke! 

Hope this helps.

Cheers, Lance 

You can use these interchangeably. It's just a different way to say basically the same thing. "To kid" someone means to not be serious, to play a joke on them, and/or to say something you don't really mean just to be funny.

For example:

Me: Wow, those jeans make you look old! Nah, just kidding, you look great! :)

You: What do you mean these jeans make me look old?!

Me: I'm joking, you really do look great.

para mi son el mismo, a lo mejor hay diferencias muy muy pequenas

I'm kidding = I'm just joking = I'm just kidding = I'm joking 

"just" puede anadir un sentido si el hablante cree que la otra persona se ofiende/se preocupa por la broma y al principio no creia que fue solamanete una broma. 

I'm just kidding = I'm only kidding = It's just a joke = it's only a joke.

"just" es mas comun (creo)