I think your question has already been answered at:
https://social.technet.microsoft.com...nal2010general
But to clarify. You do not need to set constraints on the tasks. Set the "Build Crate" task to a lower priority number (200 for example) and ensure the option to Split tasks in Leveling options is selected.
I'm also guessing you wanted the Build Crate task to occur during Task 2 & 5.