Do I need a waterproofer or something to prep the drywall above my shower for tile?

The paper will peel off of the drywall because of the weight and the steam mixed. Remember it isn't water you have to worry about as much as it is water vapor, which is just as thin as air and will go into places that regular water drops will not. If you go to restaurants most will have plywood backing, which can work in your situation if you treat the wood first with a sealer, Like Thompson's Water Seal, but your best bet is the hardy backers Or if you want to pull the tub and you have really good floors or are on concrete you could put 2" blocks behind it.