I have a 2015 all4 s manual. The differential fluid needs to be changed. And I recently watched a video that you need to replace the rear plug itself and just the crush wash in the front is this necessary? Do I need to replace the plugs each time?

 

I have an F60, on mine, the sealing O-ring is imbedded in the plug. So, a new plug has this new O-ring included. Yours sounds like it may be the same.

 

Considering that you're probably only changing the diff oil every 50k miles or so, adding $8 to the service price isn't too bad:
https://www.ecstuning.com/b-genuine-...117525064~oem/