You do not have to have a dedicated neck sizing die to neck size only. However if you neck size with a full length die then you will only be able to size part of the neck (about 3/4ths) before the full length die starts to resize the case body. If the full length die starts to resize the case body it will push the shoulder forward and can cause a crush fit if the shoulder is moved far enough forward to contact the chamber shoulder.
In most cases the way to neck size 3/4ths of the neck with a full length die is to thread the die in until it contacts the shell holder and then back it back out 3/4ths of a full turn. Somewhere around there it will size as much of the neck as possible without contacting the case body.
That being said, the Lee Collet Neck Sizers are cheap and easy to use and size the neck without introducing runout. If you had one, you could neck size until you get a crush fit and then full length or partial full length resize with your full length die. Or a better solution would be to get a Redding Body Die and then you could get rid of your full length die and that stupid expander ball.
