Sorry, I completely forgot to respond to this.
What do you mean jumping without distorting the entire map? I assume you adjust the height variable in the shader itself. Instead, when drawing an object, you could adjust the vertical position pos[1] by the height value scaled with the scale of it. So the line that draws the sprite in the Draw Event would look something like this:
draw_sprite_ext(sprite_index, image_index, pos[0], pos[1]-height*scale, scale, scale, 0, c_white, 1);
With height as the height above the ground in pixels. This should vertically displace the sprite, more the closer it is to the camera. Is this what you were after?