How to get 2D sprite renderers to cast shadows woth 3D lighting in a 2.5D game?

I tried adding using the folowing shader and I get lighting on the sprite itself, but it doesn't cast shadows on anything else.

Code (CSharp):

1. // Unity built-in shader source. Copyright (c) 2016 Unity Technologies. MIT license (see license.txt)
2.  
3. Shader "Sprites/Diffuse"
4. {
5.     Properties
6.     {
7.         [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
8.         _Color ("Tint", Color) = (1,1,1,1)
9.         [MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
10.         [HideInInspector] _RendererColor ("RendererColor", Color) = (1,1,1,1)
11.         [HideInInspector] _Flip ("Flip", Vector) = (1,1,1,1)
12.         [PerRendererData] _AlphaTex ("External Alpha", 2D) = "white" {}
13.         [PerRendererData] _EnableExternalAlpha ("Enable External Alpha", Float) = 0
14.         _Cutoff ("Shadow alpha cutoff", Range(0,1)) = 0.5
15.     }
16.  
17.     SubShader
18.     {
19.         Tags
20.         {
21.             "Queue"="Transparent"
22.             "IgnoreProjector"="True"
23.             "RenderType"="Transparent"
24.             "PreviewType"="Plane"
25.             "CanUseSpriteAtlas"="True"
26.         }
27.  
28.         Cull Off
29.         Lighting Off
30.         ZWrite Off
31.         Blend One OneMinusSrcAlpha
32.  
33.         CGPROGRAM
34.         #pragma surface surf Lambert alpha addshadow vertex:vert nofog nolightmap nodynlightmap keepalpha noinstancing
35.         #pragma multi_compile_local _ PIXELSNAP_ON
36.         #pragma multi_compile _ ETC1_EXTERNAL_ALPHA
37.         #include "UnitySprites.cginc"
38.  
39.         struct Input
40.         {
41.             float2 uv_MainTex;
42.             fixed4 color;
43.         };
44.  
45.         void vert (inout appdata_full v, out Input o)
46.         {
47.             v.vertex = UnityFlipSprite(v.vertex, _Flip);
48.  
49.             #if defined(PIXELSNAP_ON)
50.             v.vertex = UnityPixelSnap (v.vertex);
51.             #endif
52.  
53.             UNITY_INITIALIZE_OUTPUT(Input, o);
54.             o.color = v.color * _Color * _RendererColor;
55.         }
56.  
57.         void surf (Input IN, inout SurfaceOutput o)
58.         {
59.             fixed4 c = SampleSpriteTexture (IN.uv_MainTex) * IN.color;
60.             o.Albedo = c.rgb * c.a;
61.             o.Alpha = c.a;
62.         }
63.         ENDCG
64.     }
65.  
66. Fallback "Transparent/Cutout/VertexLit"
67. }