I can attest to what Oni says.
My weakness is off the floor and I find rack pulls not much easier than pulling off the floor. It seems counterintuitive, because in a heavy deadlift, once it goes past my knees, the weight flies up. But my weakness is in the upper back, which makes it harder to maintain positioning.
For the same reason, long pauses 1" off the floor have helped heaps. They tax out upper back and make you conscious of keeping your lats tight.