Before anything, delete DNF 2011 saves and configuration files, they are stored in Steam's 
userdata folder, DNF folder is "57900", after that extract the contents from DNF_Enhanced 
in the folder where DNF is installed and ovewrite files if prompted, after that run 
setup.bat and launch Duke Nukem Forever.

Also, if you're using the Direct3D 9 renderer make sure to turn on Vsync or cap the
framerate between 60 and 120, any value above 120 may break the Monster Truck speed and 
it's physics. This isn't needed for Vulkan since DXVK has a built-in fps limiter, if you 
want to edit it open "dxvk.conf" with a text editor.