What is the difference between flap load relief protection and overspeed protection?
According to this document/study guide


  Flap Load Relief – on the 757 and a few 767s, if the flaps are at 30
  and Flaps 30 speed is exceeded, the flaps automatically retract to 25.
  On most 767s, if the flaps are at 25 or 30 and the placard airspeed is
  exceeded, the flaps automatically retract to 20. The flaps will
  automatically re-extend when airspeed is reduced. If the Flap Load
  Relief fails to operate when it should, the Trailing Edge light will
  illuminate and the FLAP LD RELIEF EICAS message will display


So flap load relief is a system that specifically protects the flaps. This is the result of accelerating to higher speeds. 

Overspeed is a more general term and the actual speed that defines overspeed may vary with configuration, the 767/757 has a warning for that 


  Overspeed Warning – Master Warning, EICAS message, discrete light and
  siren when airspeed exceeds Vmo/Mmo.


It appears that overspeed protection is a function of the auto throttles.