I'll assume it's only the open end of the garage that has the leans, The rear wall being plumb. It sounds like there isn't much wall between the door and the edge of the building, no diaginal resistance. I'd suggest you use a come along fastened near the floor on one side of the big doors opening. The other end fastened as high up on the other side as you can get it, maybe to the top plate on the opposing wall. If the wall can't be pulled over to plumb easily, use two comealongs and wait a few days to re-tighten. It should relax over a week or so enough for you to pull it plumb. After it's in positing you'll need to hold it there with diaginal bracing on the interior walls at the upper corners of the big door opening. The bracing could be steel strapping. or wind bracing let into the beam over the door. The diaginal in tension will be the one doing the most work, make it as long as you can, and at a 45 degree if you can.