Its usually a pair of clutches (like a clutch pedal in a manual car). The more torque present on the driveshaft the harder the clutches are held together. This allows for some independent slip within the drive wheels. In modern cars the control of the clutch is electronic rather than mechanical, such as Mitsubishi's Anti-Yaw-Control (AYC).
See: http://en.wikipedia.org/wiki/Limited_slip_differential
http://en.wikipedia.org/wiki/Active_Yaw_Control
GM started offering Limited slip differential in 1957
The front differential is an "open" differential. No limited slip components there.
Is my 2002 Chevy Tahoe a slip differential or a limited slip differential
no
No.
No.
No.
Limited slip is a locking differential that distributes power evenly to both drive wheels.
The term "Non Limited Slip Differential" is Ford's way of telling you that your differential isn't special. A limited slip differential has clutches in it that stop a spinning wheel (the one that isn't getting traction) and send the power to the one that has traction, so the vehicle can get out of snow or mud. A non limited slip differential can only send power to one wheel at a time, which is the one with the least amount of rolling resistance (the one not getting traction).
The limited slip and Trac loc differentials require what Jeep and Chrysler sell as limited slip additive.
Limited Slip Differential 3.55 gear ratio
F-150 has never had a locking differential until the 2009 F-150 FX4 model. Only choices before the 2009MY was a limited slip or open differential.