You could argue that traction...
You could argue that traction is easily as important as ground clearance and suspension travel when it comes to dealing with tough terrain. In this situation, traction alone would be the limiting factor. Without locking diffs, this kind of crawling would be impossible.
You've already heard why four-wheeling without locking diffs is really two-wheeling, or in some cases, one-wheeling. The question is, what to do about it.
The answers depend on what you drive, how much you want to spend, and what kind of driving you do. Generally, the more you four-wheel, the more you need a traction-adding differential (TAD).
The type of four-wheeling you do determines the type of TAD you want. Almost every traction-adding differential has some kind of weakness, limitation, or downside, which might include cost of installation, reliability, handling characteristics, torque-biasing capability, or noise generation.
There are lockers, limited slips and spools to choose from.
When we use the term "locker," we mean a device that actually locks up the differential so that both tires turn equally when power is applied. These may be selectable lockers, or automatic lockers.
Selectable lockers include the ARB Air Locker, cable-operated lockers, and several types of electromagnetic lockers that can be actuated via switch. They all require the operator to decide when to lock the differential and when to unlock it, so the installation will include the need to wire up a switch or other shift mechanism on the dash. They have the advantage of being completely transparent when not in operation--driven in the unlocked mode, there is no effect on steering or handling. So selectable lockers can be used in the front differential of a 4x4 without creating a handling/safety issue. Selectable lockers actually lock up the differential to split torque evenly--usually by means of pinion gears--so they offer the performance of a true locker. And because these lockers remain unlocked most of the time, rather than continually locking and unlocking on their own, they do not add wear to driveline components.
Automatic lockers are those that lock and unlock in response to changing conditions. They default to a 50/50 torque split but are designed to unlock through corners, permitting the tires to go around a corner without scuffing, and then lock again when a torque differential occurs between wheels. The advantages of automatic lockers include the fact that no driver input is necessary, so there is no need for a switch on the dash. The disadvantages are that automatic lockers can be noisy, harsh, and they can impart unwanted handling characteristics in complicated traction settings.
When we use the term "limited-slip," we are referring to those traction-adders that bias torque away from a spinning wheel toward a wheel with traction by progressively binding up the differential between them. This may be achieved by use of clutch packs, sliding cone engagement, or gear arrangements. The advantages of a limited-slip are that they are relatively driver-friendly, without the noise and harsh engagement characteristics of a true locker, and are therefore more versatile in their application. Some can transfer more torque than others, but all have their limits. Whatever the case, a limited slip provides less traction than a true locker, but much more than an open differential.
A spool, which replaces the differential, is sometimes mentioned as an option to a locking differential. A spool functionally locks both wheels together so that they must always turn at the same speed. There is zero wheel speed differentiation when going around a corner, so the inside wheel will chirp and scuff, and hopping may occur; on pavement, the axleshafts could absorb a lot of twisting. However, spools are cheap and strong. As long as the vehicle in question never needs to go around a corner, or it is always used on loose soils, a spool could be an option. For anything other than a mud bogger or sand dragster, we would say there are better options.
A word about reliability and durability: Most lockers are stronger than the parts around them, so concerns about a properly-installed locking unit itself breaking are usually misplaced. Some tend to wear more rapidly than others, but most carry warranties of a year or more. The real risk is that in an irresistible-force-versus-immovable-object situation, lockers can transmit enough force to permit twisting up a driveshaft, crunching pinion gears, or tearing the splined end off an axle. As a rule, when big engines, torque-multiplying transfer cases and tall tires combine, it's rarely the locker that crumbles.
Following is a quick review of the types of lockers out there and their potentially ideal applications.