If the left back one is sticking, as you say, the cable could be damaged on that side.
This is the one that goes past the exhaust, check that the cable has not been hitting the exhaust, this happened to mine, and it caused the inner plastic coating around the cable to melt, and then sieze inside the sheathing.
If it is just seized becuase of crud, you should be able to free it up with a bit of force and wd40.
Just keep moving the cable back and forth, eventually the oil will penetrate and it'll free up.
- That's if the cable isn't damaged of course.
gone but never forgotten . new blue
XBOX Gamertag : celadon278
Xbox Forza3 AO : Class B joint 3rd