Similar to the front backing plates, the rear disk backing plates vibrate, and eventually crack and fall off on corrugations.

Unlike the front plates, there are no convenient bolt holes to attach a bracket to. After a few dead end prototypes, I came up with the idea of using very high temperature Teflon sheet inserted between the plate and the axle casting. 

This is part of the blurb from the spec sheet on Teflon products:

• PTFE Sheet, PTFE Rods and PTFE Tube is resistant to practically every chemical known to man and has a surface as slippery as ice on ice to which no substance will adhere. PTFE will operate continuously at 260°C in harsh conditions for long periods. It will not degrade when exposed to long term sunlight. 

For short periods it will cope with temperatures over 300 degrees, so should be fine for this purpose. There are wires for the ABS clipped to the same backing plate, so it obviously doesn't get as hot as the main brake assembly.

Sorry for the terrible photo (blame the phone!) but this is the pad I fashioned from 6mm Teflon:

The very small square of 6mm Teflon I bought cost about $20!!

It is simply riveted under the backing plate with 3 rivets with wide backing washers and it removes 95% of the flexing/vibration.  It's not as good a solution as the front plates which are extremely solid, but is will be a huge improvement over nothing.

This is a picture from the top of the rear axle, looking down:

I've got an infra-red laser thermometer, so I'll be checking out the temperatures after some long downhill braking to see what actual temperature is reached on the backing plates.