If you're rear is sitting half in a mud hole, half on slick rock, then that means 1)your fronts just came through the same mud, so the tires are wet and/or caked, and 2)the weight of the truck is on the rear axle ('cause you're sittin in a hole). That front, even with locker, isn't going to have enough traction to pull you out.

I'll use an SAS Pathy for an example:

In picture 1, you'll see the situation:
Picture 1

In picture 2, you'll see the rear axle "results":
Picture 2

Notice, the weight is on the rear axle, one side is in the mud, another side is up on the muddy bank. The front is out of the hole completely. This particular Pathy is locked in the front, and open in the rear. Notice how well his front "pulled" him through the hole... The vehicles that were locked in the rear, and open in the front, didn't have any problems with this area, and drove right through it.

Lock the front first, and you're wasting your time and $$. 'Nuf said.