You may remember, Dave, when you were a participant in my rolling gantry development a couple of years ago… and the difficulties I was having then trying to eliminate the image skew I was seeing with the early “free-rolling” version of my foam-ripper gantry. Over the next 40 or so posts in that thread… you, me, Jamie, and Jeff had quite the discussion (and it was fun!) as to how/why we were seeing an incredible sensitivity to the “levelness” (or lack of) of my machine. After considerable beating of my head against the wall, I finally concluded that if you want a machine to accurately move over a worksurface, it needs to be referenced somehow to – and not independent of – that worksurface. In that case, I was able to simply re-belt my machine with LR-style belts down the edges of the worksurface to provide the “reference” and it completely eliminated the skew. It was, of course, no longer “free-wheeling”… but was now useable.
The magnitude of the skew I was seeing might be “insignificant” in large chalk drawings on a sidewalk or parking lot but I think it still might have relevance to what you are seeing, “…the wheels on each side do not end in the same place at end of print.”
I don’t want to discourage you in any way from going down that rabbit hole… I think it will be both instructive and fun. I really am interested to see what you end up doing with this but felt I should at least gently remind you of some fun discussions we had “way back when…” 
– David