I've actually taken a different approach to fix this problem. I won't say its limited to Y, but Y (at least one mine) is very flexible and therefore easy to get "out of square". You can think of it as a mass-spring system where the mass is the spindle stuck into some material. As you move further away from the Y motor, it is going to apply an increasing torque force to the Y axis.
On mine I an shake it with my hands which is probably way to flexible. My approach (unproven at this point, since I didn't get the parts yet), is to use some gussets to square up the Y axis and apply additional contact points between the Y cross member and the motor plates. This means I'll have to take apart the "X" plates to drill some more holes for the new gussets. In addition (and since it is easy), I am going to add another square piece of Al to stiffen the motor mount where it attaches to the cross member and my new gusset. Ideally I'd do it on both sides, of the plate, but since it is asymmetric, there isn't enough room.
A couple of drawbacks to this approach:
1) it will cost about 30mm or so of Y travel (25mm on one side, and 5mm on the other side).
2) There will be an extra moving mass and unfortunately it is high up on the system which could cause it to vibrate more and become less stable as it accels/decels.
3) It still may not be stiff enough but I am limited on space in where I can add gussets.
I can't do anything to address #1, that is just the way it is. On the other hand, if you are thinking of adding a drive on the other side, you are going to burn some travel there as well since the idler will collide with the Y carriage. On the other hand, the extra drive is lighter and lowers the mass-center, but it is probably not very stiff.
For #2, I am having a couple of different flavors coming of varying strengths and weights. This should allow me to find the best one mix of strength/weight/vibration. I am partial to stainless, but high up on the Y axis might not be the best place for that. It will end up being 12 bolts, washers, nuts, and 6 fairly hefty gussets. I estimated the added mass at 180-240g for the hardware and another 90-100g for the stiffening on the motor mount plate.
I have a couple of ideas for squaring up and stiffening the X axis as well, but I haven't figured out the ones I want to try yet. For the moment, the Y axis is probably the one that needs the most stiffening.