Center Differential?

Hi, my name's Jamin and I'm a newby.

Alright, I looked in owner's manual for information about the difference between 4H and 4HL and it said that putting the transfer case in the 4HL position locks the center differential. Now, just what the crap is the center differential and what does it do?

The center differential is within the transfer case, and 'locks' to ensure that the front & rear driveshafts rotate at the same speed.  What this does is split power evenly, as opposed to acting like an 'open' differential, which sends power to the wheel with less traction.

Think of it this way...when you round a corner, the 'outside' tire actually travels a further distance than the 'inside' tire.  In order to turn smoothly, a gearset must allow the axle shafts to turn at different rates...hence, the term 'differential.'  The center 'differential', if unlocked, allows the front & rear ends to rotate at different rates.  When locked, they spin at the same rate, to ensure bite and power distribution even if one set of wheels loses traction.

o.k. that makes sense. That actually makes alot of sense. But do all transfer cases have a center differential? (not just lockable ones) Also, what's the advantage to not locking the center differential?