I also think the diff is most likely given the noise has just started suddenly. A wheel bearing wil gradually get nosier unless the bearings suddenly collapse.
I have similar miles on a D4 3.0TDI without any issues but i dont have the sports diff. My previous D3 had one rear wheel bearing replaced after a similar droning noise but it was at about 150k miles.
If you are out of warranty, it would be worth having the diff oil changed, condition of the old oil might indicate if the diff is in trouble. Shouldnt cost much to have it done.
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