Whining at Highway Speeds

Put my 92 VR-4 away for the winter and everything was fine. Took it out in the Spring, and noticed a kind of low-pitched whine when at highway speeds. Replaced both Transaxles because I have previously been told that the bearings were wearing.

Still have the same sound. Seems to be coming from the rear end of the car, so I suspect maybe a differential issue.

Could I be low on fluid and could this cause the problem, or do you suspect that it could be something else.

Car has 95K miles and engine and tranny seem very strong.

ya i get the same thing but its just around 55mph