load a DLL reference from a different folder?

If the DLL is in a sub folder you can add this folder to the AppDomain private path.

The private bin path of an AppDomain cannot be changed once the AppDomain has been created (AppDomain.AppendPrivatePath is obsolete), if you don't want to create a new AppDomain, you can modify the probing element in the App.config.

If it's not in a sub folder, things get more complicated, you can load the assembly using its full path, but if it references other assembly, the CLR won't be able to resolve the dependencies.

If you want to resolve dependencies, you can add an handler to AppDomain.AssemblyResolve and fetch the needed assembly in your specific folder.

Another possibility is to place this DLL (and dependencies) in the GAC.

If the DLL is loaded dynamically, just specify the full path to it. If it's static, or you feel like it, add the library path to the PATH environment variable. Yes, this DOS relic is still around.

Also, read the doc on the LoadLibrary() API for a better understanding of DLL loading.