TD4 found reason for bad vibration

Hi have been having a bad vibration from 1300 - 1700rpm on my TD4....(bad in 4th & 5th at 30/40mph) engine goes well and is willing.....but I had this boomy vibration I could not source. Well, on my truck I had different tyres front to back in pairs. (as with the car when I bought it)....Landranger services thought it would lead to trouble .... with the IRD ..... so I measured tyres and found a 5mm mismatch in diameter!!!
So I changed the front tyres so they were all the same.....and bingo car felt no different....but 90% of vibration has gone! I can only think the 4wd set up on these waggons is real sensative......maybe you get a weird harmonic being generated that effects the dual mass flywheel......so to all you effected check your tyres and perhaps try changing front to back...

So now you need to move the new ones to the rear.....

Rear ones are only one month old....so the diameters should be the same....

I've got different tyres on mine back to front, the fronts look newer but when I measured the rolling circumference the rears were actually bigger by about 5mm (in circumference, not diameter).

I was thinking of changing them around as I have some vibrations but after I measured them I was reluctant to do so as then the slightly larger ones would be on the front.... I might try it anyway to see if alters the vibration - might show an unbalanced wheel if nothing else.