2004 Mercury Mountaineer rear axle and differential

Did repair on rear in differential an passenger side rear axle differential fine rear axle keeps breaking at the boot closes to the differential housing repaired twice in 2 days its a 2 door 4.0 liter 3.55 gear ratio did notice little round plate that bolts to differential didn't get put back could that throw rear end out of alighnment or what would cause rear end to become unalighned