is it possible to fit and use a KX250F monoshock in a KLX250S?

 

What's the eye-to-eye length of both shocks? It's almost certain that the geometry of the linkage and how it affects damping are very different. If the shocks are the same length and you can get the eye bushings to match the KLX, it's likely going to need revalving at the least. I'm thinking a late model KX shock is going to be longer based on the longer travel, but I don't have one in hand to check.

I'd be inclined to revalve the KLX shock either yourself or by a source that performs it. The rear shock on the KLX is actually pretty good and responds well to a revalve. It's the fork that's the weakest link.