shaders - 2D HLSL World position

I'm trying to get world position from my vertex shader to my pixel shader so that I can disable the shader once a preset X coordinate has been passed (no shading once I'm over X).

Getting the screen position is not a problem so far but despite my best efforts to look after it and implement examples the calculations just don't return the preferred world positions I'm looking for.

Update: So got it to somewhat work, after compiling the shaders the output changes to such:

Could anyone explain why this happens?

I should mention that I'm really new to HLSL, been only scripting so far.
Edit:Added matrices.

world = Matrix.Identity;
view = Matrix.CreateScale(new Vector3(1, 0.75f, 0)) * Matrix.CreateTranslation(-playerpos.X, -playerpos.Y, 1); 
projection = Matrix.CreateOrthographicOffCenter(0, view.Width, view.Height, 0, 0, 1);
Matrix halfPixelOffset = Matrix.CreateTranslation(-0.5f, -0.5f, 0);
projection = halfPixelOffset * projection; <\code>

    texture lightMask;

    sampler mainSampler : register(s0);
    sampler lightSampler = sampler_state{Texture = lightMask;};
    float4x4 World;
    float4x4 View;
    float4x4 Projection;

    struct vs2ps
{
    float4 Pos : POSITION0;
    float4 TexCd : TEXCOORD0;

    float3 PosW : TEXCOORD1;
};

vs2ps VS(float4 Pos : POSITION0,float4 TexCd : TEXCOORD0)
{
    vs2ps Out;
    Out.Pos = mul(Pos, World*View*Projection);
    Out.TexCd = TexCd;
    Out.PosW = mul(Pos, World);
    return Out;

    }

float4 PixelShaderFunction(vs2ps input) : COLOR0
{   float2 texCoord = input.TexCd;
    float4 screenPosition = (input.PosW,1.0f);
    float4 lightColor = tex2D(lightSampler, texCoord);
    float4 mainColor = tex2D(mainSampler, texCoord);
    if(screenPosition.x < 3500)
    {
         return (mainColor * lightColor);

    }
         else return mainColor;
}

Answer

Okay so, finally achieved my goal.

view = Matrix.CreateScale(new Vector3(1, 0.75f, 1)) * Matrix.CreateTranslation(-playerpos.X, -playerpos.Y, 1);
world = Matrix.Invert(view);
projection = Matrix.CreateOrthographicOffCenter(0, view.Width, view.Height, 0, -1, 1);
Matrix halfPixelOffset = Matrix.CreateTranslation(-0.5f, -0.5f, 0);
projection = halfPixelOffset * projection;




texture lightMask;
sampler mainSampler : register(s0);
sampler lightSampler = sampler_state{Texture = lightMask;};
float4x4 World;
float4x4 View;
float4x4 Projection;


struct vs2ps
{
    float4 Pos : POSITION0;
    float4 TexCd : TEXCOORD0;
    float3 PosW : TEXCOORD1;
};

vs2ps VS(float4 Pos : POSITION0,float4 TexCd : TEXCOORD0)
{
    vs2ps Out;

float4 worldPosition = mul(Pos, World);
float4 viewPosition = mul(worldPosition, View);
    Out.Pos = mul(viewPosition, Projection);
    Out.TexCd = TexCd;
    Out.PosW = mul(Pos,World);
    return Out;
    }

float4 PixelShaderFunction(vs2ps input) : COLOR0
{   float2 texCoord = input.TexCd;

    float2 screenPosition = input.PosW;
    float4 lightColor = tex2D(lightSampler, texCoord);
    float4 mainColor = tex2D(mainSampler, texCoord);
    if(screenPosition.x < 3500)
    {
         return (mainColor * lightColor);
    }
         else return mainColor;
}

Like this the visual output doesn't change and I get the correct coordinates back.

Thanks to those who tried helping tho, they actually set me on the right path.

I'll leave this here for future reference in case anyone's struggling with the same.