has it done the idle since then? Could have been gunk built up on a port or sensor or the like and when you revved it up it moved the gunk out of the way.

The clunk could have been a couple of things. The tcase may not have completely disengaged and you could have a front axle shaft ujoint or driveshaft ujoint going out.

As far as full time or part time for wheeling in sand.... I would probably use part time to have the same power bias front/rear. I don't think the full time would be necessary on the sand as it would on the road where the traction would vary from sticky pavement to slick water/ice.