I have a 1997 D-90. While I realize the truck is in full time 4-wheel drive, Im a little confused about when to lock the differentials. When I go over soft sand for longer drives I usually air down the tires to about 15psi and lock the diff in high by moving the lever all the way to the left. On short stretches of sand I usually just leave it in the normal diff-lock off position. This past week I needed to drive over about 100 yards of soft sand. With diff lock off and full air in tires I got about 25 yards and started to bog down. I put diff lock on (high setting) and slowly accelerated; nothing. Wheels not moving. Aired down the tired and tried to move the truck in diff-lock off position. Still bogged. Move the difflock into lock high engine spun for a few secs and with a thunk it engaged and I was able to back out onto the road again. Mechanic said I should never have to lock the differentials unless Im really bogged down deep in something. Would appreciate your opinions.