This is the rear brake assembly mounted to the rear axle.
To remove the backing plate the whole axle shaft needs to be removed. Begin by dismantling the old brakes. See here for a detailed step-by-step process.
Once the brakes are off, you can get to 4 mounting bolts holding the backing plate to the axle – through the access hole in the wheel rotor.
Don’t forget to remove the brake lines from the middle of the rear end (on top of the differential) into the slave cylinder.
Once these 4 bolts are off the axle shaft will just slide out. Note that there is no need to go into the differential to remove the axles as my Chiltons manual suggested.
Now the axle is out, the backing plates will slide off.
Install the axle shafts back in the rear end, to prevent damage.
Here is what that work was for: