[KiwiBacon]

It's difficult to sift through the BS sometimes to see what each system actually is.

Basically there are two main systems.

Part-time 4wd.
This runs in 2wd (rear wheel drive for a pickup truck) and the front wheels can be engaged mechanically (use a lever), electrically (switch) or pneumatically (also usually controlled by a switch).
When in 4wd they don't like being used on hard surfaces, you scrub up the tyres and risk breaking an axle, cv or other drivetrain parts.

Full-time 4wd.
This is in 4wd the whole time and has a centre differential to drive both front and back wheels without the hard surface problems that part-time 4wd vehicles have when in 4wd.
There is usually a way to lock the centre differential to make front and back axles turn at the same speed when traction is likely to be lost. But a lot of modern vehicles rely on computer control of the brakes (traction control) instead of locking differentials.

Then we have the variations of these systems.
A lot of four wheel drive cars drive the front wheels the whole time and switch in the back when needed. This can be a computer controlled clutch or as simple as a coupling which allows enough slip to stop the problem of breaking stuff (i.e. a viscous coupling).
Some makers have a full-time 4wd system but with a centre diff that will only allow a certain amount of slip. Some also split the torque differently front to back.

Some makers (mitsubishi comes to mind) have a system that used to let you switch between fulltime 4wd and part-time 4wd. Why I have no idea as the fuel economy wasn't much different to full-time 4wd.


Some cars have enough commonality between 4wd and 2wd versions where it's possible to simply bolt the 4wd drivetrain in (I think I own one like that, but I wouldn't know until I actually tried to do it, could be lots of small differences). But other times everything can be different including the body not having clearance for driveshafts.