You could also try a retina iMac too. I've owned quite a few monitors and I've had a few that were not simply not compatible with my eyes. Something about the panel gave me awful eyestrain, but other versions of the same monitor didn't.
As @velocityg4 suggested, I'd check to make sure that the settings are all the same. Check to see if the color temps are the same. Differing color temperatures can also make eyestrain worse. If the retina is more blue than the non-retina, try adjusting the color temp so that it's warmer. That trick has helped me before in the past with a panel that was killing my eyes.