Q: Do I need a passport to cross back to the United States?

Let's ask that a different way:
Q: SHOULD I have a passport when crossing back?
A = Well, yes... you should.
It makes things so much easier and it is what a citizen really ought to have,
and it makes the border agent purr like a cat..

But getting to the action:
Q: Can I get by without a current passport?
A = Yes. Yes you most certainly can.
Trust me - read on.

Here's the straight poop: your border agent prefers that your butt be passported
when you come through their lane.
But you are going to be fine with a driver's license and a sunny, respectful attitude.
If you want to go an extra length, make a copy of your birth certificate.
It is overkill and they may not even look at it, but it demonstrates respect and scores a brownie point.

The border agent is legally bound to allow you into the country if they believe you to be an American citizen.
So unless you act a fool or pose a threat, there is no passport "jail".
You are not in trouble and you won't be detained for more than 90 extra seconds
while the agent sweats you about why you don't got no passport, if he or she is in a mood.
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Maybe they will be perfectly sweet and say nothing.
They might hand you a pamphlet on how you are a passport sinner and how to get right with God.