Have you checked the keepers on the backside of the Z guidways to be sure they're ok?
I agree 100% on the casting could be the root of your proble. Be sure to use a machine level and check both direction while keeping the table centered on the Y axis. Move from one travel limit to the other and stop in the middle of travel as well and adjust your pads to maintain the level bubble is centered in the X axis as well as the Y axis. You can amplify the pitch, roll and yaw thats in every machine if the pads are out of adjustment. I used to use a 90 degree magnetic level on the Z axis guideways after the able was level to adjust the pads. Place it on the side of the guideway and the front as well, and adjust the leveling pads on the column. Be sure to use quality machine levels to do all of this with.
I suspect that the excessive wear on the gibbs and probably the keepers as well will need to be addressed as well. New turcite and properly scraped in.

Good luck!