Have you checked your rear bushings? After time they tend to wear down to an oval shape insted of a circle. It may be possible to loose 1 inch if the bushings are sacked because it lowers the rear arms pivot point
If it's not very close then you may have problems like the others are suggesting.
Even when the ride height is dead even at all four corners, Zs appear to be leaning backwards. This is somewhat of an optical illusion because a) the front wheel wells are larger than the back wheel wells and b) the slope of the rear hatch.