Do you have one, I did & thought as it seems everybody else does that it is the A frame ball joint or worse, however I had to replace the old pattern exhaust with a new one yesterday & noted how close to the rear body the pipe that loops over the axle was,so made an effort to ensure everything was pushed up tight & in the right place, low & behold, when driving today...No Clunk.. It was the old exhaust fouling the body. Footnote it may only be on early Disco's as origonaly they had a 2 part system which was apparently a bugger to get out & so pattern exaust's apeared with an extra joint betwixt the boxes.
Chris
1990 Disco 200Tdi