Key thing with rack pulls is to make sure your hips are back and shoulders over your knees. If someone is looking at you from the side then they ought to see a triangle formed with the three sides being your torso, your arms and your thighs.
back is in natural position, i.e. thoracic extended, lumbar with natural arch, scapula retracted and depressed. Feel the weight in your heels as you set up for the pull, brace your abs, tight grip and pull 'er on up. Make sure your hips and shoulders are moving at the same speed. Keep the weight close to the body, coming up the thighs and drive with forward with your hips. Remember, go after hip extensions and not hyper extension of lumbar area.
Many times when performing a dead lift or rack pull and one feels it in their low back it could be because the weight is too far out in front of the body or your just not sitting back enough to allow the glutes and hams to take over.
hope this helps