anyone here go to a chiropractor?

Pull ups and pull downs are good for shoulders and upper back, back lifts are awesome (gyms have machines for these - they support you with a bar under your abdomen and you bend down towards the floor and back up again like a reverse sit up).

If you don't have a machine for back lifts just stand up, put a bar over your shoulders (no extra weights at first) and just bend forward at the waist and then back up. Do it slowly and concentrate on feeling the muscles in your back clench.

Once your back is pretty solid, start with deadlifts. Put a weighted barbell on the floor in front of you, stand up straight, and then bend over and pick it up. Slowly straigten your back so that your arms are down by your sides and holding the barbell. Then bend forward and put it back down. This is a difficult exercise depending on how strong you are and how much weight you use.

The Superman is good as well. Lay on the floor on your stomach with your arms and legs outstretched (like Superman flying) and bend upwards, lifting your arms and legs off the floor. Hold for two seconds and release. You should really feel that in your back before long.

The lower back group (the posterior chain) is difficult to isolate, but deadlifts and back lifts target it along with the other back muscles. To really thrash it try some unilateral leg lifts or the heinous Bulgarian split squats. Look those up for pictures or diagrams. Those are a bit more advanced though, and not truly necessary for your purposes.
 
Kiwi: I love chiropracters, they've helped my fix messed up back problems twice now. I went to one up until 7 months pregnant with Haylee.