2011 Ford F-150 FX4

[Senor_Taylor]

A true 4wd usually will not have a diff in the middle. If you have a diff in the middle it means that the front and rear both receive power all of the time, with some slip I'm between.

4wd usually implies that you are either 2wd or 4wd. You have to choose. When you are 4wd, there is no slip because there is no center differential. It is a transfer case, which does not allow variable speeds between the front and rear.

On an awd, you would not leave the center diff locked just like you wouldn't leave a truck in 4wd on dry pavement. In both cases, the front and rear can not differ in speed and it causes driveline bind when turning. Think about a welded rear diff and how it doesn't allow you to turn. Now imagine instead of one axle, it's the whole car.

Over simplified version of everything and is not 100% correCT:
4wd is either on or off. 4wd or 2wd. No center diff, instead has a transfer case that has no slip. No speed variation.

AWD is always on. Has a center diff, so no driveline bind. Not as good as 4wd in snow or mud because of the slip in the center diff. Some AWD vehicles have a locker on the center diff. When this locks, you essentially have a 4wd. You can not drive normally on pavement as the driveline will bind.

The terms "part time 4wd/AWD" and "full time 4wd/awd" are thrown around to mean a lot of things and it's not always the same. So don't worry about that, just worry about whether it has a transfer case or a center differential. If the latter, pay attention to whether it locks or not.

Don't worry about the rear diff locker. That has nothing to do with anything. Literally any rwd vehicle can have a diff locker.

Edit: DJ's edit is exactly correct. We are saying the same thing.