OK. The rear axle is basically a pipe, we call it a beam. Anyway, there is no adjustment that your alignment shop can do. If the suspension, trailing arms and springs are OK, then the rear "axle" needs to be inspected to make sure it is not damaged. This is what the shop that did the alignment is saying, that the rear axle is damaged, Right?
You will need to make sure that it is not bent. If that is why the shop wants to replace it, then you need to replace it.
I do not recommend making any modifications (plates, shims or eccentric bolts) to a bent part. If the rear "axle" is not bent then the most likely cause is the springs are either not the original springs, someone lowered the car or the springs are weak.
There is a measurement that needs to be made. Measure from the center of the rear wheel to the rear fender lip (directly vertical). The distance will be, between 352 mm and 382 mm. At the lowest number the camber is allowed to be -2 degrees and 10 minutes. At the highest number (382mm) the camber is allowed to be -1 degree and 10 minutes.
You can see the big difference ride height plays in the camber settings.
If your springs are bad you will be wearing out the inside edges of your rear tires.
The shims that are installed on the rear axle are for performance (race) cars. I do not recommend them for your car.
Measure the rear ride height like I explained above and let me know.