ok here goes lol
after a while of the brakes moving on the backing plate, the tabs the contact the backing plate wear grooves into it. when you take the rear shoes off, check the backing plate and see the blocks that the shoe edge contacts it. i bet that there is a little groove wore into the backing plate. i have seen this so many times. there are a few ways to correct this.
1) weld the groove level and grind them flat
2) if they arent too bad, you can just grind them flat
3) get a new backing plate
always use a high temp brake lube on these contact points
almost everytime i came acroos this problem, i have just used a die grinder and made the flat and smooth again.