Lack of lubrication is the number one cause. The suspension sits on pressure points within the components. There is no grease in these areas as the weight of the car has pushed it out of the way and is resting metal on metal. When lubing the chassis and suspension, take the load off of it by jacking it up rom the body. This will change the geometry and allow the grease to reach those areas normally under load. Also keep a lookout for the indicative "orange dust" around any ball joints, bearings, bushings, u joints and hinges. This is a sure tale sign of a dry component. The orange dust is actually fine metal powder from the metal on metal contact that has then oxidized. Break out the manual and find EVERY lube point. Check them off as you go along. All it takes is that one hidden one that has been missed the last 20 years!
I know its a chore but disconnecting the shock and compressing the spring abit will make it much easer to diagnose the seized component. You wont be fighting them while working the suspension up and down with a pry bar.