Tire noise?........

For the past year my '89 3.9 4WD has been making a noise in the rear that sounds like bad tires, but I don't think that is the problem. The sound is a whining sound that gets higher and lower pitched as I speed up and slow down. The transmission has been fine, so I don't think that is the problem.
It almost makes the same sound as when it is in 4wd, but it is not in 4WD.
Any ideas?
Thanks,
Dave

check the fluid level in the rear end. It would be a good idea to change it, if it hasn't been done yet.