It can be checked very simply.
1) set disco to off road height
2) jack up offending corner
3) grab hold of wheel and wobble/shake. Look for movement in all bushes
4) if step 3 doesn't show fault place on axle stands and remove wheel. Using a pry bar manipulate all bushed joints until offending bush is found.
5) offending bush will most probably not be a service item and the entire wish bone will need replacing should it be found at fault.
Had this problem on my disco. Replaced front wishbone in about 5hrs (first time I'd done it) and will be rusted solid. YouTube discovery front wishbone removal for full details....
Main stealer wanted £900 to do it. I did it for £300 (including parts, tea bags, loads of plus gas and cutting disks for grinder)
Hope that helps
