Perhaps somewhere along the drivetrain there is a link that can be adjusted. Are the two driving pinions on the same shaft, extending through the gantry, or is there a shaft coupler somewhere in the mix? Crap walking sounds like a bad idea.
There are ways of using marking compound to determine just how the pinion is utilizing the rack. If you search some rack manufacturers, you should be able to find it. The marking compound would wear off unevenly.
Wish I could look at that thing up close, there has to be some kind of adjustment built in, or perfection in the mounting of the racks.