Update 2019-04-03: Had to reprint part in PC+ filament since PLA was warping. Added V3 version with tighter tolerances and more supports to prevent warping.
Update 2019-04-01 : Added V2 version with a spot for the nut in the small piece so you can turn it into a brace that pulls together instead of pushing apart if your wheels go the other way.
It seems that some factor causes the carriage wheels to point in towards the Z stepper motor for some reason. I'm thinking either the weight of the carriage and time or else maybe adjusting the clamping pressure bolt on the Z tube causes the wheel to turn.
Anyways, this fixes it. First square up your carriage, then adjust pressure on the Z tube holders so that all three rollers are touching the rod gently but you can still make the wheel slip by turning them with your finger. Then put an M3 bolt and nut in this piece, push it all the way down over the ends of the wheel bolts and turn the M3 bolt until the wheel is parallel with the other. I find a metal ruler against the wheel bearing a good way to check.