As long as all 4 are on the ground. Pick one up (usually the rear end) and the opposite one, as in other side other axle.. will spin also..
example, you put your front right tire on a big rock, and the front left falls in a hole, the trucks weight will be (mostly) on the front right, and rear left. a little on the front left (the front end is slightly heavier then the rear) and the right rear will be in the air.
the tire hanging in midair will spin, and the front left (in this example) will also, as it has a few hundred lbs of weight, trying to pull a 5000 lb truck.
I got board one day on a jobsite, crossed a ditch lifting my rear left 18" off the bottom of the ditch. after getting out I could almost pull the truck down and rock back & forth, which tells me there was 2 to 3 hundred lbs on the front right. I was able to drive out sence the front was pointed downhill. With my new calmini bumper on the front, it is way heavier, i doubt i could rock it even with 1 other persons help.
If I were going up hill, i would have needed to be yanked out by the guy i was doing the survey with. The (almost) open diffs are useless when you unload a tire. A rear locker would have kept the truck moving (or dug a hole) reguardless of how many tires are on the ground. If you are in a position that you need to turn, or hit somthing are lift a tire, either winch out, or you need 2 lockers. just the rear and you (should) move, you may not have much steering with one front unloaded, and the other not pulling.