1975 Mercedes Benz 450sl wheel locks up

If it was okay before, we can rule out mechanical problems. The most likely cause is rust buildup that formed on the brake drums from humidity. That will wear off eventually. Drum brakes are much less effective going in reverse, so you should be able to back up and apply the brakes to wear that rust off. Going forward that rust can grab the front brake shoe and make a brake self-apply.