2002 Kia Pregio Van / noise at rear on high speeds.

We recently replaced the rear diff about 3 months ago. Purchased second hand, from wrecker. The reason why we replaced it was because the noise was gradually getting louder as it lost more diff oil from the flange seal, located on propeller shaft. Whilst driving you could not hear much noise, but as soon as you let go of the accelerator the noise was very loud. Like a grinding noise. Anyway, once we replaced the diff everything was back to normal. For three months everything was running smooth. Now all of a sudden, a similar noise is apparent only on speeds above approximately 80km's and above. But this time the noise is more evident when the van is placed in neutral on speeds above 80km's and above. You can just barely hear the noise whilst in gear but soon as you place the van in neutral it is damn noisy and you can just start to feel vibration in the steering. Does this mean the diff we bought is on its way again or could it be a wheel bearing? When we had replaced the second hand diff we filled it up with new diff oil and there are no signs of leakage. We also checked the level of diff oil, ok. Please help us find out what the cause may be.