Unless you have some sort of major back problem, you have got to be doing something wrong. Deadlifts are a very safe exercise. One big AHA for me was to arch my back at the very bottom,when I'm setting up. Sort of think flaring your butt at the guys playing for the other team (every gym has them). Keep your weight balanced evenly over both feet, and--most importantly--keep a solid contraction in your abs all the way through the movement. This is in all likelihood what you are not doing. The way I think of it is you are doing nothing other than bringing your hips to the bar, and the only way to do that is to stand up. If your stomach is loose, your hips won't move, and your swayback will win.
As far as barbell versus dumbell, if you're using the same weight, I would tend to think the dumbells are actually better. Dumbells are just much tougher to load. I tried getting a 150 lb. dumbell out of my gym rack once, and it wasn't pretty.