The fact that you have to back up to unlock it makes me think the problem is in the parking brake, possibly the equalizer bar between the shoes. Make sure all that stuff is loose and not binding on the shoes. Maybe the metal on the new shoes is too thick and the shoes expand outward but then get hung up on the equalizer bar and won't return by itself. Reversing might push that shoe back in on the equalizer bar and lets the shoe return. Just for fun when it locks up set the parking brake and release it and see if that unsticks it, it might also make it worse.