The systems are completely different and the Torsen front diff has nothing to do with what makes selectable 4wd and awd different. Full time 4wd functions almost exactly the same as awd but sounds more rugged for marketing purposes. Our trucks use a transfer case where as awd vehicles have a (sometimes lockable) center differential. Having awd actually does much better in situations like snow and ice where you don’t want power to be locked 50/50 front to back as one axle might have more traction and it also allows you to keep stability control and brake vectoring traction control on. The advantage of 4wd (Or a locked center diff in an awd vehicle) is that it forces distribution of power and in a situation where all 4 wheels have very limited traction, an open center diff in an awd vehicle will struggle putting it down.
The torsen front diff simply replaces the pre-facelift open diff in the gen 1 and allows both front wheels to have power in extreme low traction situations.
Some vehicles have a mixture of both. The Mercedes G Wagon has a lockable center differential whereas the 4Runners use a Torsen center differential and a lockable transfer case. Torsen stands for “Torque Sensing” meaning they function as a limited slip differential.
