Discovery 2 rumble

Hi All , looking for some advise on a rumble I have on my D2 TD5 manual at speed over 50mph ( gets louder the faster you go) , I was convinced to begin with that it was coming from a rear wheel which had shed all its balance weights.......However no, I then put a fully balanced set of alternate wheels and tyres on but still the same .Since then both props have been balanced ...still the same , There is no vibration only a speed sensitive rumble any thoughts would be appreciated also it doesn't sound like a wheel bearing

many thanks John

id be tempted to lift al 4 wheels off the ground and try it through the gears till you reach the relative wheel speed, prop balance is very unlikely though the wrong phase or poor uj is possible ,is the damper on rear of t/box secure

Thanks James will give that a go

Also check the condition of the rubber doughnut; the flexible coupling at the back end of the rear prop shaft.