Can I use a threaded rod for a rafter tie?
Corrosion would be one concern.  Another would be carefully and slowly tightening the turnbuckles in a sufficient and even manner.  Your rod could well deform the wall/roof if adjustments are done too quickly.  Small adjustments over months.  Perhaps a laser and mirror could give clues.  

I used SS cable with stainless fittings for my leaning garage and used 2 levels to put a leaning wall straight.

Many issues regarding fixing leaning, sagging buildings are here