Crack shot adds +2 to critical damage and when you get righteous fury against vehicles you get to roll 1d5 on the crit table (RoB pdf slide 170). If you have crack shot does that mean you get to roll 1d5+2? The argument to say no is that RoB says that the d5 is not "damage" and crack shot adds "damage."
If the answer is yes that makes this skill phenomenal against vehicles. If the answer is no then I'm having a hard time understanding how this ability is worth 500xp to a devastator. Given the 20+ damage range on most of their weapons the odds of ever having the +2 make a difference seem extremely low.