Do I need to replace all my diff fluid out when replacing the front wheel actuator for 4 wheel drive

My 4 wheel drive isnt working so I want to take off the front actuator to see if the motor is still good. I looked at videos on how to do this and ive seen you loose fluid when you take off the front actuator. So im assuming you would need to replace the diff fluid. Would I need to drain and replace all diff fluid or could I put the amount that came out back in?

 

If it hasn’t been changed, change it. If it was changed recently, just top it off.

 

I'd test it from inside the truck.
https://www.tacomaworld.com/threads/4wd-system-operation-help-thread.742420/