What age Disco? If a Disco 1 they use almost the same same steering. The D2 isn't all that different either.
Lots of things to check, first place would be to get some one to rock the steering wheel back and forth (engine of and key in the ignition to remove the steering lock). You'll probably be able to hear something clonking, while some moves the steering wheel back and forth (fairly vigorously), crawl under and see if you can where the noise is coming from. You may have worn balljoints/track rod ends. Ot you might be able to see/hear movement in the drop link from the steering box. You can also see the box from the top if you open the bonnet.
Other areas to check are the radius arm to chassis bushes and radius arm to axle bushes. As well as the wheel bearing and ensure the hubs/swivels are properly affixed to the axle tube (they can work loose on occasion).