Perhaps not directly answering the question, but important info nonetheless - backing plates require the removal of the rear hub/bearings. This is, depening on your OOF Top Trumps Experience Level ...
1.0 Pah! I do them in 15 mins a side between making love to my three wives and learning the Violin
to
9.9 That is impossible, I spent two days with the car up in the air, snapped two pullers, nearly broke my wrist, eventually had to drive to a garage to get them to do it - who failed. Where's Serek/Marks DTM/Nick W/JamesV6CDX? ? ?
Basically you need some custom tools to make the job doable. It's not impossible, and
is doable with the right tools, time and prep. But it's not half as easy as replacing the fronts, that's all.
The second option is to weld on replacement patches to 'professionally bodge' the holes which have appeared. Long term option is, of course to strip down, refurb and repaint the rear axle assembly, with new hubs and backplates. Depends how far you want to go financially and time-wise.