Differential rear end for a trike? It isn't simple, but not terribly complicated either. Take a normal rear hub, and spoke flange mount two identical sprockets that have had the centers removed to allow clearance (you can do this with an internally geared hub, or very carefully with a derailleur set up). On the internal end of each rear half axle mount a freewheel (you'll need both a right side and a left side freewheel), then put identical sprockets on each. Chain loops connect the rear hub and the two half-axles. When power is transmitted through the hub to the half-axles, the freewheels engage and turn the wheels. When you coast, or are powering through a turn where your outside wheel needs to turn faster than the inside wheel, the freewheel allows that to occur - effectively acting as a differential.
It actually works pretty well, according to what I've read, and the local bike shop guy who says he has done this a few times on various trikes.