Amro,
There is no way to share .NET references between LabVIEW and TestStand. You can pass a .NET reference from one to the other as a handle, for the sole purpose of passing it back later, but you can't use it in the other environment.
The reason this is true for .NET, but not things like ActiveX, is that .NET creates it's own universe to live in - it's managed code (C#, VB.NET, etc), unlike LabVIEW, TestStand or ActiveX, which are unmanaged (C++, VB, etc). Thus both LV and TS have their own ".NET universe", and the references don't map between them.