Chances are whoever you scored it from had the same issue?

Sounds like some kind of alignment issue.

Rebuild and replace seals and bushings.

Could be the upper and lower clamps are torqued out of alignment somehow -- with the wheel in the slider, undo the pinch bolts holding the stanchions at the upper clamps, then re-torque to spec.

When you have it completely disassembled, use a flat surface to double-check that the stanchions aren't bent.

On that same flat surface, check that the lower isn't torqued out of plane as well.