As mudrunner said, test engaging in your drive way is ok while wheels are straight, but you need to move a bit for everything to line up and engage. Until it engages, the difflock dash light will flash and become solid once its engaged.
Same is true when unlocking. You will more than likely need to be moving and off the throttle, and the dash light wont go out until it has fully disengaged.
Sent from my SM-G965F using Tapatalk