There are many available for Toys. The ARB Air Locker is a highly-regarded locker that is manually operated via a switch that controls compressed air that locks up the diff. When it is off, the diff behaves exactly like an open diff. When you press the switch, you have a spool and its awesome traction. This diff is expensive and hard to install, but having manual control is very nice for a rig that spends time both on and off the pavement.
Automatic lockers operate on a 'ratcheting' principle.
Neither wheel on the axle can turn slower than the ring gear, but they can turn faster. Automatic lockers take some getting used to when turning - the inside wheel powers your truck around the corner, while the outside wheel is 'freewheeling'. With an open diff, the outside wheel does the powering. Automatic lockers are cheaper than the air locker and provide the same traction, though there is some ratcheting noise and wheel hop on the freeway. They can cause some spooky steering on slippery surfaces.