Loud whine

It would depend ono where the noise is coming from ans since it is speed related, I would start with checking the various differentials. They could be out of gear oil.

Other possibilities would be wheel bearings.

Why can't you shift into neutral while driving?I have done that many times on many different vechiles for diagnosing problems.

https://www.2carpros.com/articles/checking-fluid-levels

Above link has information on how to check differential oil level.

For checking wheel bearings, lift up wheels and check for excessive side play by shaking the wheel lightly. Turn the wheel manually and note if any noise ie emitted. Light noise would be hard to detect if you do not have any experience with it.

So are you talking about the front differential or rear differential?Also you can put a hand on the coil spring while turning the wheel to feel for a bad wheel bearing.

Check the front of the differential, item # 27 in diagram. The link is generic and location sometimes differs.

LUBRICATION

Capacity is 5.5-5.8 pts. (2.6-2.7L) of Motorcraft SAE 75W140 High Performance Synthetic Rear Axle Lubricant (F1TZ-19580-B). If vehicle is equipped with Traction-Lok, add an additional 4 ozs. Of Friction Modifier (C8AZ-19B546-A). Capacities listed are approximate. Fill differential until fluid level is about 1/4" below bottom of filler plug hole.

There should be a side level fill hole level to fill and check the level.

Wheres it leaking from?

Leaks are usually from sels and those can be replaced but if it is due to worn bearings, replacing the sels would not work. A rebuild is necessary.

It sounds like the throw-out bearing or input shaft bearing is out. Can you please shoot a quick video with your phone so I can hear the noise? That way I can tell for sure what's going on. You can upload it here with your response.