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, yea... you should.
It makes things so much easier and it is what a citizen really ought to have,
and it makes the border agent delightfully happier.

But getting to the action:
Q: Can I get by without a current passport?
A = Yes. Yes you most certainly, absolutely can.

Here's the straight poop: your friendly neighborhood border agent prefers your ass be passported
when you come through their lane.
But you are going to be fine with a driver's license
and a good, sunny, respectful attitude.
If you really want to be a good do-bee, make a copy of your birth certificate.
It's overkill, but it demonstrates you are respectful of your lack of passport.
Scores brownie points.

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.
​
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.