It's interesting because it is suggesting that the springs are equal, and that the difference (cause of the sag) is that the sagging side's axle end is aligned to the frame too far forward, stretching the spring on a forward angle.
Dealing with these often, the plastic hammer is not always enough, but the threads are very at risk.
I usually don't take the nut all the way off, I bring it back just to where the top of the nut is flush with the end of the bolt part of the arm. It protects the threads when hammering. Then you can use a piece of wood and a better hammer and give a good whack.
I find it odd that they don't mention looking into the other side also being off (nut on too much in comparison to the other)
You can use those same washers from the front that mercedes sells for the back I think....I have always just machined them or used other stock from my drawer.