Wow, a quarter inch is HUGE!!!
Without seeing your gantry, I’d guess that there is far too much slop between the bearing and the associated angle iron.
This is a “test as you go” dimension. Again, I’m guessing that the Linear Bearing Mount (orange in the image below) needs to be ‘longer’, such that it pushes the bearings tightly to the base.

YOu may need to make a replacement, you might be able to shim the bearings.
The fit needs to ‘pinch’ these two parts together (see red arrows) tightly, so the axis still rolls smoothly AND there is no slop between any of the parts.

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