How to build a limited slip differential? Lego provides a couple of ways for building a differential, such as this:



and also provides a limited-slip gear like this:



http://www.brickowl.com/catalog/lego-technic-differential-with-one-gear-28-tooth-bevel-with-open-center-62821

What's the simplest way to combine the two mechanically such that if you have one motor driving two wheels, they can turn at different speeds (i.e. they aren't locked to each other), but at the same time, they will both receive a reasonable amount of force (i.e. if one wheel does not make contact with the ground, the one that does will still apply some force, as opposed it it not spinning at all and the free-hanging one going at twice the speed)? 

The simple solution is to just use some belts to drive both wheels, but I would like a geared solution if possible
If you haven't seen it, Sariel's limited slip differential technique might do what you want, but it's a little different than what you are asking for:



This setup doesn't provide a clean way of producing a fixed amount of torque to a wheel if it's partner is running free. It instead simply disables the differential if the wheels have been running at different speeds for too long. This isn't a perfect solution, but it is very simple and may work for you depending on exactly what you are trying to accomplish.