Thanks for this, Gustavo. The examples in your link show how "need" can be used as either a true modal or a normal auxiliary verb. The website stops just short of telling us that, as an actual modal, it need not be inflected, whereas it needs to be inflected when used as a simple auxiliary:
C1: No one under 21 years of age need apply for this position.
C2: No one under 21 years of age needs to apply for this position.
I would say that the difference in meaning is significant.
PS: Yes, I know. I did it on purpose. Sort of. Don't you find that the modal form is almost exclusively used in the negative?