My driver's seat did the same thing when I bought my car. I took the seat out, turned it upside down and found a spring to be broken. I unbolted the seat bottom and my father took it to his work place and got the broken spring welded and bolted a small split sleeve around it. The repair is holding up well 1 year and 3 months later.

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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold