discovery td5 juddering problem

hi guys im after a bit of advice i got my 1999 disco td5 chipped a couple of years ago straight away the clutch was slipping so i fitted a brand new clutch kit then a year later my transfer box went bang so i replaced that with a new reconditioned one 3 months later the transfer box went bang again and destroyed every thing inside the transfer box so i had a garage replace the transfer box as mine was under guarantee now since that has been done i get a juddering mostly when in 3rd gear when at 30mph and about 2000rpm does any one have any ideas on what it could be cheers regards mark

At a wild guess, I reckon the tranfer box is liable to go bang at any moment