Spring rubbers are cheap, but a totally temporary band-aid.
Airbags are a last resort.
In order of preference, here the three main routes to consider.
OPTION 1: Find out what two, custom made, heavy duty rear coil springs would cost.
Probably a lot less than you think.
There is bound to be a spring repair shop somewhere in Milwaukee. Find it. (Local NAPA or a truck repair shop will know their name.) A lot of (maybe most) spring repair shops only handle leaf springs. Even if that's the case though, they'll know who works with coils springs.
You'll need to know:
- wire diameter
...Size of the rod/wire/round stock the coil is made from.
...Basically this is the only measurement that will be different on the new springs.
- coil diameter
...Don't recall if it's inner or outer diameter they'll ask for.
- number of coils/windings
- uncompressed spring length
...Preferably specifications from a manual.
...Alternatively measured from an UNCOMPRESSED, new/oem spring.
...Yet more alternatively measured from an UNCOMPRESSED, apparently unsagged, used spring.
P.S. While you've got it apart, consider reinforcing the suspension arm. Do it yourself, but ask the guys at the spring repair shop if they think its necessary.
P.P.S. CoyoteX suggested Metro front springs are similar to rear but taller and might work. They might, but could be a bear depending on how far the suspension arm drops once
the strut and knuckle control rod are disconnected. (Or whatever the last little bit is before you r&r the spring.)
OPTION 2: Booster springs (obtained from salvage yard)
inside existing springs.
This diagram from Autozone
is for a '89-'93 Metro, but it looks reasonably similar to
the picture you previously posted.
If you disassemble far enough to get the springs out, you can weld seats for a smaller coil spring to the upper spring seat (#2) and lower spring seat (formed as part of the suspension arm (#18). The main caveat to
mounting a smaller coil spring inside the main coil spring? To prevent interference during compression, the inner coil spring windings should be aligned opposite to the outer spring windings. e.g.
Outer spring laid on its side - ///////////
Inner spring laid on its side - \\\\\\\\\\\
OPTION 3: Boost springs (obtained from salvage yard)
on struts/shocks
This assumes the scuff mark on the shock/strut in the previously cited picture isn't due to insufficient clearance. Muffler Clamps and large, home made washers could be used as upper and lower spring seats.