Hi alex8199,
Thanks for your question. If you want to shuffle all of the songs in a single playlist on your iPod Classic, you need to turn Shuffle on for Songs. This will shuffle songs in any album or playlist you choose.
To do this choose Menu > Settings > Shuffle. Press the center button to toggle your options and choose Songs. Press and hold the Menu button to return to the main menu.
Now, toggle to your playlists, choose a playlist and press play. You should see the shuffle icon in the top right corner of your screen and only the songs in the chosen playlist should be playing. If you don't pick a specific playlist, all of the music on your iPod classic will be shuffled.
Alex, take a look at the iPod Classic User Guide for additional information.
Have a great day!