I'm not smart enough to comment on the "causes" of this common problem, but my 
car had it when I first bought the car - and that was with less than 30K miles 
on it.  I had the rear spring re-arched, and now, with a bit under 65K miles, 
it only sags a little.  Or at least much less than before and certainly not 
enough to worry about.  But I do like that GT6 spring idea.....  Some day 
maybe.....
FWIW/YMMV/etc/etc.
--
George P. 
NASS #290 
'78 Spit 1500 
Ruther Glen, VA
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