Everybody - As long as we're on the subject of brakes, when is the last time you flushed the brake system and replaced the fluid?
Brake fluid is hydroscopic, meaning it attracts and holds moisture.
This is why big trucks sometimes lose their brakes while going down long grades. When the brakes heat up due to extended braking, the water in the brake fluid turns to vapor (steam), which is the same as having air bubbles in the fluid. (i.e., the same as brakes that haven't been bled properly.) Experienced truckers, especially those in mountainous regions, use (engine) compression braking to help keep their speed under control by downshifting.
On long steep grades, instead of just trying to keep my speed down to 50-60 mph, I brake harder to slow down to 40, 30, or even 20 mph. Then, I release the brakes for a while (and allow speed to build up again) to allow the brakes to cool a bit before applying the brakes again.
The hydroscopic nature of brake fluid is why the cans have those frustrating metal/plastic seals under the caps. This is also why I dispose of cans of new brake fluid that have been opened for more than two or three months. I only buy quart/liter cans of fluid when I'm flushing and replacing brake fluid in a vehicle. Otherwise, I buy only the small cans by the case so I don't have to throw out a lot of new fluid that's been open for a while.
In cold temperatures, the water in the brake fluid can freeze and the ice crystals can block the lines and valves, preventing the brakes from working.
Alternatively, the water can cause corrosion in the system, which can build up and plug the lines and valves. (Bits of rubber from old, disintegrating flex lines can also build up and plug the system, which is why I also prefer to replace older flex lines, especially when I'm replacing the calipers/cylinders.)
BTW - Do NOT, repeat NOT, use alcohol to flush the brake system -- it WILL damage the seals in the system, leading to brake failure! Only use compressed air or, preferably, fresh brake fluid.
PoorGary - In your case, corrosion or ice crystal plugs could be keeping your calipers from releasing. In a few cases, when I've been "reviving" older trucks, I've had to use compressed air, blown backwards through the lines, to break loose contamination plugs. (This is a temporary fix -- I always replace all the brake lines when I encounter this situation.)
Try jacking up your front wheels, one at a time, and see if you can turn the wheels. If the wheels turn freely, your problem is elsewhere.
In interior Alaska, people never use the "parking" brake. (It's an emergency brake, not a parking brake.) In subzero temperatures, "parking" brakes tend to freeze, locking the rear brakes in the engaged position. This will keep a vehicle from moving. (Voice of experience, from twenty years ago ... and it took over four hours of careful heat application to thaw them without damaging anything.)
Instead, use wheel chocks if you don't feel the transmission parking cog is sufficient.
Alternatively, your torque converter could be going/gone out. (You have been changing your transmission fluid and filter regularly, right?) The symptoms you describe indicates this is a possibility. (Again, voice of experience, although not due to neglected maintenance.)
BTW - A518 torque converters in older Mopar vehicles tended to do this due to substandard material in the lockup clutches. (More recent and aftermarket torque converters usually have better clutch material.) In those cases, the disintegrated clutch material tended to plug up the transmission coolers, destroying the transmission by causing them to overheat.
