Is your rear diff's pinion seal leaking? If you own an Amarok the answer is going to be yes at some stage, it's science, like gravity or Nickelback still being awful. You'd think that throwing a new pinion seal would fix that bad boy right up right? Wrong! Well, sorta wrong.
Y'see, if you use your 4WD as a 4WD and drive in terrains like sand, dirt and mud, chances are your pinion seal has allowed some grit into your rear diff flange, and just like sand in your undies, it can wear a groove in the flange's surface. What this means is that as soon as you throw your brand new seal in place, that flange-groove will still allow oil to escape. That's where the speedy sleeve climbs softly into bed with you and strokes your hair and tells you everything is going to be just fine you little champion.
The sleeve effectively re-walls the flange (which is a $500+ part from VW) and allows your new pinion seal to do to your diff what world governments have done to Julian Assange - i.e. keep things leak-free.
So if you're tired of your diff looking like Clive Palmer on a treadmill and want that sweaty mess to go away, you know what to do. Throw the problem at AIS and let us make it all better and let you get back to cutting sick hoops at Killy Loop like the boss you are.
*Disclaimer: Please do not cut sick hoops anywhere. You're better than that and you deserve your family's respect goddammit