Try this:
Drive forward while turning to the left. Go for at least 1/4 of a turn; this gets all the driveline wound-up in one direction. While still moving, reverse the direction of turn. At the same time place pressure on the 4wd lever to disconnect. At some point in the turn, the driveline will be trying to go from "wound-up left" to "wound-up right" and will have no wind-up on it at all. This is the point at which the pressure on the lever will pop it into disengaged.
Of course, it makes no difference if you start out going left or right--all that matters is that you reverse direction.