I've heard Cholla Cactus Garden and Sheep Pass are good for sunsets, but I haven't tried those spots myself. And a lot of photographers like Keys View for sunsets -- it's a beautiful view across a huge valley.
When I was at Joshua Tree last spring I found much more fodder with the post-sunset skies than I did with the sunset event itself. But that's just me.
There's an article about shooting at Joshua Tree
on my website, if you're interested in any more info.
Have a great trip. Joshua Tree is one of my favorite national parks.