Last night it rained quite a bit here and today it got a lot colder. I tried to drive my 06 6MT today and the rear wheels are locked even with the parking brake lever all the way down. What can I do to maybe try to unfreeze it?

 

The rear brake is a combination caliper for the foot brake, with a drum/shoe for the emergency brake. Most likely, the emergency brake is what's stuck.

How bad is it stuck? If you give it some gas, will the car move? There's an adjustment on the rear E-brake, but the wheel has to be in the right position to gain access to it.

Read about it here (scroll down to Parking Brake Check and Adjustment:
https://acurazine.com/forums/3g-tl-2004-2008-93/brakes-caliper-o-h-pad-repl-hoses-bleed-disc-specs-park-brake-adj-3g-garage-d-041-a-578017/

Your other option is to jack up the car, and try to pull the rear rotors (won't be easy with the shoes binding). Then remove/replace/fix whatever's seized up.

 

It will move a bit but it will make a loud screeching noise, which I don't like. It's snowy here and I don't have time to fix this my own. Hopefully the dealer can take care of this because it's still under factory warranty.

 

UPDATE: I sprayed some de icer on the back of the rear disc and still nothing. Only the driver side rear wheel was locked up even with the parking brake lever all the way down. The dealer told me that they just cleaned off the rust on it and it's fine now. If the parking brake mechanism doesn't loose up because of the rust, doesn't it need to be completely replaced?

 

The parking brake is a drum brake "inside" of the disk. It's separate from the rear caliper.

I would drive it a few feet at a time and see what happens. The "regular" rear brakes have a bad habit of calipers sticking.

You could mark the disk in several spots with a sharpie and see if it gets rubbed off quickly after a couple rotations. This will determine if it's the caliper that's binding or the parking brake shoes.