Back brakes squeal very loudly reverse only

Hello,

It sounds like the rear pads are possibly glazed and need replacing.
Replacing the pads are a straightforward process and can be done in a short period of time, depending on your comfort level.
Keep in mind that anytime you replace the pads it is recommended that the rotors get resurfaced (given that they are within specification).

Here is a helpful guide:

https://www.2carpros.com/articles/how-to-replace-rear-brake-pads-and-rotors

I have attached diagrams for your reference.

Please let me know of any questions.
Thank you.