The 424 replaces the stock front driveshaft with a "selectable" driveshaft. The 424 transfer unit enables you to disengage the front wheels from the drivetrain and reenable them when you want.

 

Instalation is pretty easy. The directions are easy to follow, also. The hardest part was breaking loose the bolts that hold the differential. I had the old drive shaft out and waiting before I even got the 424 in the mail.