whining from rear of car

[twan]

i've had my supra for a month now and i ever since i had it i hear a mosquito type whining from the back of the car while driving around 44mp/h (70km/h). i think its the differential, because before i went to buy the car the previous owner said he had to take it to the shop to fill the diff. then on the way home i smelt oil burning and drove it straight to the shop to get certified and mechanic called me back to tell me that the burning smell was the diff leaking on the exhaust and said it had to get rebuilt but to save me money he just replaced the leaking oil seal.
is it possible that before i bought the na supra the seal drained the diff and wore out a bearing or gear to make it whine while driving?

[jvginpdx]

Have you checked to see how much oil is in it, or if it has any?