My biggest concern is my lower back, which has a degrading disk (just about my tailbone) from spending 30+ years on a surfboard. When I sit up straight, no pains at all...hunched over, not good.
Mark, you know your body best but most riders I know with back issues say the cruiser position (back straight, arms and legs out) hurts them the most. Of course, all back problems are different. I would urge you to spend time seated on each bike you're considering, with hands on the grips and feet on the pegs. Give it some time.
I have very minor back issues and they are aggravated by an upright riding position because my spine winds up taking a vertical pounding as the miles add up. I don't like the crouching sport bike position because it's difficult to see in the city, so I look for sport/standards. These bikes have a relatively upright seating position with relaxed legs. But, your arms are outstretched and your back is arched forward a bit. This also helps aerodynamically at speed. If you're fully upright, you turn into a sail at speeds above 65 or 70.
There are other reasons I don't like cruisers, but that's why there are so many different bikes out there. Hell, I personally think cruisers are an abomination, but whatever gets your juices flowing is what you should go with.
Before you plunk down the cash though, you really owe it to yourself to take a look at the new BMW F-650GS. The 2009 models have the new 2 cylinder 800cc engine. It has the perfect riding position for you and every sort of option (storage, heated grips, ABS, etc) known to man. Fully equipped, it will be out of your price range but it's worth the comparison. Good luck!