Y dose back tires lock up

Try backing off the self adjusters and see what happens

If it backs up okay we can probably rule out an axle problem. A common cause of drum brake lockup is a leaking axle seal. Gear lube gets on the shoes and drum and makes a sticky mess. If you're on a dirt road or driveway, the clue will be only one rear wheel skids. Both might if you have a locking rear differential, (positraction was GM's name for it).

Ford has also had way more than their share of parking brake cables rusting tight. If one is just sluggish, it will not release fully under return spring pressure. That will hold the shoes out and make them grab easily. Most commonly that affects the left rear brake going forward and the right rear going in reverse on those Ford models where the right cable goes all the way around and comes in the from the back.