If you are shifting the Tcase and you pause in neutral, it will grind.
Take the load off the driveline, put the clutch in or shift the auto to N and be either stopped or moving very slowly. Then make one movement of the T case shifter to 4 low. Don't stop in Neutral. A clunk will probably be heard and that's fine.
Of course you can shift directly from 2 hi to 4 hi while driving, but not to 4 low.
When you pause in Neutral on the transfer case, you give the gears a chance to spin up and there is no synchro to help match them during the shift, like in the transmission. Just clunk your way on through.
If you do spend too much time in Neutral and want to avoid the grind, shut the engine off and complete the shift after a couple of seconds.