Have the wheel alignment checked. I'll bet if the car has been hit, it's probably off. A difference of one inch is usually not visually evident. If it looks off then you might check major body sheet metal alignment in the rear. Bushings, etc, would really have to be wiped out and noisy to give you enough sag to be visible and generally both sides would be affected. The weak spring is a good bet if the other stuff checks out. try bouncing the rear of the car and see if you hear any groans, clicks or other funny noises. You might also have a bent spring seat. Many possibilities back there. Good Luck

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Jeff L
1987 300e
1989 300e