The spring preload has nothing to do
with the rebound & the lack of it
is why the back is bounceing theres
no control on the shocks return stroke..
from what you say it sound like the return
damping has all but gone...
XT shocks are pretty nasty at the best of times..

If you budget can go the distance
Ohlins 46DR1

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Harley Davidson..
The easyest way to turn gasoline into noise with out the side efect of horse power...