Sorry gambit but i'm agreeing with VR6 on this one, if the sping cups go then you'll have a knocking noise as its these that hold the spring in place. The rubber wont fail completety it will just start to crack and fall to bits.
Rear springs dont fail, and shocks dont sag that much, especially rears where there is v.little weight
Well no.
The rear springs sag and will do over any given length of time due to them losing their compliancy, weight of vehicle and journeys will make the springs sag.
the shock absorbers will go and this will only make the car feel skittish as your not correcting the re bound of the spring. sometimes the dampers seize but it is rare to happen.
Also thanks for the picture of the rear dampers, as I haven't got a clue what they look like and have had my eyes closed everytime I have ever removed or refitted rear suspension.
the easy solution is look at the rubber, if its knackered replace it.