There's a whining sound at speeds between 20 mph-50 mph. It sounds like it's coming from the driveline, but I had the universal joints replaced at the sound is still persistent. The rear differential is full of fluid. Could this be the rear diff going out or maybe the transfer case??

You might want to have the fuel pump checked. They make that whinning sound when they get older. My brother in-law's 98 has been whinning for a few years

Before you worry about your transfere box or diff check your wheel bearings,

That may be possible. Clean out the area and check again so you would see better where exactly the leak comes from. Of the sound is loud or similar to metal, that may be it or a bearing assembly may be going out.