Backing plate is optional. Don't worry about it.
Noise sounds like U-joints unless some rusty thing is rubbing on another rusty thing. The version you sent me sounded like potatoes in a garbage can. See if you get a noticeable clunk shifting forward/reverse. Sometimes there will be some rust dust around the joint if it's eating itself.
Wonderful. Assuming U-joint, should I be able to replicate the issue if I jack up the rear of the car and give it some gas?