Yes, all four have traction or the axles are rotating at the same speed.
Don't you think if there would be a problem engaging the CDL at any speed the Toyota engineers would have designed in a safeguard or a warning sticker? i.e. the factory front and rear lockers won't engage over 5 MPH or...
I think you are overthinking this. There is really no issue locking the CDL at highway speeds as long as wheels are not spinning. Just make sure the dash indicator light works properly so you know when it is engaged.