I’m not sure if you’ve look at this yet but it might help.
http://stronglifts.com/deadlifts-lower-back-pain-injury-technique/
As he says start the lift by pushing through you heels not lifting with your back.
Sorry I didn’t comment on it when you posted your video but here is something I thought. It seemed to me that you were arching your back (up) to much like you were trying to over compensate so that you wouldn’t round your back. You should strive to keep your back straight throughout the lift, even when you lock out at the top.
In the video it looked like you were lifting 135 lbs. Does your back hurt at that weight or only when it gets heavier. I ask because at heavier weights your form could really be breaking down in a way that doesn’t show at lower weights.
When do you feel the pain, is it throughout the lift or only during a certain portion of it?
I would advocate that you try lifting sumo style since it lessens the strain on the lower back while increasing leg involvement. However, even if you do lift primarily sumo style you would be wise to still include conventional deadlifts since they will help strengthen your lower back and are actually probably better for overall hypertrophy. But consider that conventional deadlifts done properly shouldn’t cause lower/middle back pain so switching styles could just mask an underlying problem.