If you have a hi lift jack, place the foot on one side, and the arm on the other side and ratchet away, that will straighten on the tub easy. If your issue is just on one side of the tub, use the same method, just put the "good" side up against your garage, if brick, and then use your truck bumper as the stand side, then lever away....not the prettiest way - but works extremely well!!
If you dont have one - borrow one - or buy one - You will need it one day!!