I had a vibration at 25mph and a faint judder around 45-50mph...
I drained my Oil and changed the filter in January - and fingers crossed is vastly improved....still a tiny judder approaching 50mph, but I think that may just be me 'looking' for it....so not a Cure, but will certainly a) Help you diagnose the condition of the box by looking at the oil condition and b) help out slightly....mine isn't 'cured' by significantly better and much much reduced...
Dead simple to do, I wrote a How To in the How To section with photos detailing the procedure....
Only 'Special Tool' required is a Torx Bit set and a method of refilling the Box afterwards (I used a fence Pressure Sprayer)...and note, make sure you can undo the fill plug BEFORE you drain it out...as it can be bastid tight....
Linky to the How To....
http://www.landyzone.co.uk/lz/f71/zf5hp24-fluid-drain-filter-change-212722.html