Here is my take and experience with anything with drum brakes. Your brake shoes usually contain some sort of organic compound which will swell when exposed to moisture. You aggravated (unknowingly) the situation by washing then parking.
Here is how I have fixed this in the past,as you have already tried acceleration through the stuck brakes. Take the wheels off of the rear. With a dead blow hammer or metal headed hammer, tap around the outer edge of the drum. DON'T GO SWINGING FOR THE FENCES HERE. In conjunction with drying the strikes will help to loosen the brake shoe/drum bond.
Rocking the vehicle back and forth can loosen the drums however you are putting a large amount of stress on the inner components and in time they could fail prematurely.
Best of luck, and don't freak out.