I think you will know when you get to the crossing. If the hardpack road is well traveled then the sand is probably not going to be a problem. I almost never use 4 low. Just put it in 4 high when on the dirt and drive thorough. If the truck starts to slow down give it some more gas. You will be able to tell if you're in trouble as the truck will start spinning the tires and rapidly slowing to a stop. If the tires are spinning but you're still going forward and not digging down then you'll probably make it across.
If you come to an unanticipated stop. Or you can tell that you are not going to get to the other side then stop before you dig in any deeper and air down to about 15 psi. The worst thing you can do is keep spinning the tires when stopped and dig them into the sand. Yes airing back up with a small compressor sucks and you need to shut it off and let it rest from time to time. But that can be done while sitting in camp, not out on the trail. You're not going to hurt the tires driving them 10mph down a dirt road and it will greatly smooth out the washboards as a bonus. But know if you have to air down, that includes the trailer tires as well.
Also know that most 4 wheel drive trucks are really only 2 wheel drive, the back right (passenger) side and front left (driver side) tire. Get out or have your wife get out and observe which tires are spinning when you try to drive and those are the ones you will want to put the carpet under if needed.
