Frode H. Haaland wrote this excellent article with pictures on Adjusting the Swivel Pin Preload on a Land Rover Discovery.
He covers what preload is, diagnosis to see if it will effect the steering on the Disco in question, and then walks you thru the whole job, and even provides extra notes, a parts and tools needed list, and cost estimates.
If you have owned a Land Rover for a while, you will have heard "swivel pin preload" more than once. What this item is and what it does, is really simple - but amounts to loose and imprecise steering if it is not within specified values. This is caused by wear, is usually rectified within an hour, and doesn't cost anything! Learn all about it!
What is this preload?
There's two swivel pins each side - one upper and one lower. Pictured here is #12 the lower swivel pin, and the item marked "NSS" is the upper swivel pin of an ABS-vehicle, the #33 is the one for a non-ABS equipped vehicle.
The wheel has these swivel pins as their axis of rotation when steering.
If this rotational movement is too easy, i.e. it can move freely without any resistance, the vehicle will tend to steer on its own as it meets imperfections on the road - like humps and bumps, and you will have to counter this self steering with your own steering wheel.
Thus, there obviously needs to be a bit of resistance to hinder this movement. But not too much - or it will increase the wear of the swivel pin and its bearings (or even make turning the steering wheel a bit tight..). The preload should be within a specified range.