There are two square adjusters on the back of the backing plate. Turning them moves a cam in an arc which pushes out on the shoes to press them against the drum. Adjusting them likely won't help as the wheel cylinders are likely stuck so the shoes won't be able to move/retract. It would still be worth a try as you never know, it may be able to move but the problem is you can't see which way you're moving the shoe when you turn the adjuster. You can try turning the drum while turning the adjust to see if it will move easier in other positions.
In any case, you should still be able to get the drum off. Since it's a cone shaped hole on a cone shaped axle, it requires you "shock" it off. You need a big puller and BIG hammer.
Best method I know is to take nut off, then put it back on backwards, turning it only far enough for the nut to be flush with the end of the axle. Then put on the wheel puller, and place a soft piece of iron between the wheel puller shaft and the axle shaft for a buffer so you don't mushroom the end and whack it good and hard and they will come off.
