People need to show me what feats ROTJ Luke has that suggest he has better TK defense than Kenobi. Or how he's magically going to have a better defense against Dooku's lightning than one of the greatest swordsmen in the Order's history who specialized in fighting DS warriors; Sora Bulq. If they don't there's no reason why Dooku won't force crush Grievous like Mace did and then snap Luke's neck through the force, or electrocute him.
Hell Sidious' half assed lightning was too much for Luke to block. The Count's lightning while not Sidious level has the capacity to kill Force sensitives like Night brothers and Kiffar warriors. I'd bet that Sidious' initial burst of lightning was weaker than Dooku's maximum.