Could it be the power steering unit or the rack & pinion?

You need a mechanic. It's their job to figure out what is needed without resorting to wasteful trial and error methods. Whining is normally a sign of air in the system from the fluid level getting too low, but you said the level is okay. Lack of power assist is usually the fault of the rack and pinion assembly but it almost always affects turning in just one direction first. That is more common in cold weather when the teflon seals on the internal spool valve shrink, but it clears up in a few minutes when the power steering fluid gets warm. That problem gets progressively worse over time. An even bigger problem is your vehicle doesn't have a rack and pinion assembly. It uses the older-style steering gearbox. While problems could develop inside it to cause a loss of power assist, that is not a very common failure.

Except for a few Chrysler products, replacing rack and pinion assemblies is way too involved for the average do-it-yourselfer, especially if the vehicle isn't on a hoist. It's not smart to replace a somewhat high-failure part with a used one that is likely to develop the same problem. Steering gearboxes are considerably easier to replace but you'll need a special puller to remove the pitman arm. That CAN be very difficult to do.

If you see fluid moving inside the reservoir, there is likely a valve broken or sticking inside the pump. That can cause a buzzing noise. For the lack of power assist, mechanics use an inline pressure gauge with a valve. If pressure builds to around 1100 psi with the valve closed momentarily, they can determine if the pump is working. Without having that test equipment, the pump is the most likely suspect.

Be aware too that way too many people confuse a lack of power steering assist with a lack of steering ability. You can still drive it without power steering, but at lower speeds you're going to need both hands to turn.

It is common on any brand of vehicle for power steering fluid to be pushed out of the reservoir when the pump is not pushing it back into the system. That happens when turning the steering wheel repeatedly with the engine off, with the drive belt broken, or, as I suspect in your case, when something inside the pump is broken.

Rather than guessing, I always feel better seeing test results, but since I don't have a pressure tester of my own, I would just install a different pump to try it. Power steering pumps are relatively low failure items so I wouldn't hesitate to use one from a salvage yard. The front pulley bolts on with three bolts. That makes removal WAY easier if it has to come off to change the pump. If it doesn't, it will typically be included with a used pump. Check that the diameter and number of belt grooves are the same as on your old one.

If you can't figure out how to get the old pump off, I prefer visiting a pick-your-own-parts salvage yard where I can experiment on their cars. If you're anywhere between Ohio and southern Georgia, there is a real nice chain of yards called "Pull-A-Part" where you pay your buck and throw your tool box in one of their wheel barrows. You can spend all day there. Customers and employees are real friendly, and the yards are all very clean and well-organized. You can do an inventory search on the internet too to see how many vehicles they have like yours.