Could be that the previous owner had the back of it loaded down often and bottomed it out from time to time? The stock rubber bumpstops on the axle are now old and not very soft. When you hit them it normally hits pretty hard. I bought softer progressive rubber/urethane ones for the front of my truck. Now if I bottom out it nicely "squishes" them down instead of a direct hard impact.
I know they have oil filled shock absorber type bumpstops, and have also seem some small coil spring types. Different color springs meant how stiff/hard they could take an impact.
Basically just something to progressively stop the axle from moving upward is my guess.
You don't jump it a lot do you?
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