Whining noise above 35

Please help, My 98 Cherokee Sport has developed a hollow whining noise that only occurs between 35 and 50 mph. It sounds as though it comes from the rear of the vehicle, I thought it was tires, but new tires have not solved it, I added oil ti the rear diff and the transfer case. Any ideas would be greatly appreciated. Thanks

How are the rear axle bearings? Had this in a Ramcharger once and it turned out to be the right rear axle bearing.

I vote for bad axle bearing...I had this happen on my 84 XJ. It gets louder and louder, if you turn left and then turn right you should a difference.