I'm definately not the most qualified to help, but:
When does the clonking happen? Is it all the time, when taking up drive, overrun, braking etc.
I don't know what sort of Disco you have, but if it's one with an LT77 gearbox it might be that. Chock the wheels and climb underneath with the handbrake off and the car in gear. Grab the rear prop shaft and try to turn it. Before I replaced my gearbox I could get a full 90° of rotation.