The rear differential in question is an electronically controlled limited slip differential with locking mode. The electronics control a hydraulically actuated clutch pack which divides the torque between the rear axle driveshafts. This is very, very different from a traditional locking differential (which has two modes: open and locked). It is also very different than a limited slip differential (LSD) which mechanically limits on the difference in rotational speed between the axle driveshafts (and which has the speed differential is defined by the physical design of the components of the differential). It is also very, very different from an electronic limited slip differential (or pseudo-LSD) which uses the ABS components and software to apply a braking force to a slipping wheel.
A rear differential with an electronically controlled, hydraulically actuated clutch pack with locking mode (like that in the 3.73 rear axle installed on 4th generation Expeditions) is designed to give one the best of all possible worlds. Like a locking differential, you can force the axle drive shafts to rotate at the same speed no matter how much traction each tire has. Like a traditional LSD, you can the rotational speed differential between the two rear drive shafts. And like a pseudo-LSD, you can respond to vehicle sensor data and vary the torque between the axle drive shafts. But unlike a locking differential, you can allow some (but not infinite) speed differential between the two axle driveshafts. And unlike an LSD, you can choose whatever speed differential you want. And unlike a pseudo-LSD, you can actually lock the rear differential.
So, in the 4th Gen Expedition, how does the rear differential actually operate? Well, that's what the drive modes are telling the system to prioritize. Set it in 4A, and it's going to assume you're driving down a highway and it's going to act like a lot like a traditional LSD or pseudo LSD. Choose "snow" and it'll likely do really cute things, like act like a locking LSD when you're stopped but also watch your speed and yaw sensor* to keep from putting you in the ditch....
The downsides are cost, complexity, and heat build up in limited slip mode.
*Note that the center transfer case in these fords is also a hyrdaulic clutch pack. So "icy/slippery road" mode is also going to be varying the center transfer case clutch pack to keep you from spinning round and round should you hit a patch of black ice, which was the bane of 4H 4x4 systems.)