Ok was'nt sure as you just changed the esc and that would effect balance. Yes a dirty rough diff can cause the car to pull but its not the only thing. Really I'd suggest you go ahead and rebuild the diff as your planning and then hit the track for a test. If you still have the pullcheck all the pivots, check that the frontend is smooth and nothing grabbing or binding. Also did you check that the rear axle is perfectly centered and that it has the correct end-play? Either of those can cause a pull, if all those are good it really should drive arrow straight. Again as you dont know the condition/age of the hocky sticks I'd recomend you order a new set. Thay work great but are a wear item and do need to be replaced every so often.