When "software engineer" and "upper trapecius contractures" are written side to side, they powerfully ring a bell.
This is quite common. Many hours seated, hunched, with no attention to your body because your mind is debugging subroutines... Cycling won't probably help (your legs may be moving, but you are seated and hunched over the bicycle...) but swimming should.
One important point is that, no matter what sport you do or what muscles you strengthen, you will still have problems if you don't pay attention to what is causing the problem, the rest of the day, when you are out of the gym/swimming pool/whatever.
This is one of the points mentioned in this fantastic series of three or four posts you should read: "Deconstructing Computer Guy" in T-Nation, I think they will give you a better understanding of what is happening and what may be a better approach. I quote a paragraph. After mentioning some sporting activities (spinning among them) and showing a picture of some guys riding stationary exercise bicycles in a gym, the text reads:
The vast majority of those exercises promote a forward flexed position
or compressive/shear force on the spine [Read: not good]. By repeating
the same posture in the gym that he was in from 9-5, computer guy WILL
produce improper motor patterns that just cause or reinforce all of
his problems in the first place!
Also, if you use linux, there is a wonderful little free program called Workrave that will remind you every X minutes (by default I think it is around 5 min) to make a micro-pause and correct your posture, and once every hour it will remind you to walk away from the computer, do some light stretches and rest a few minutes. I t will periodically remind you to do some randomly choosen light exercises (like some arm circles, fixing your sight in the distance and so on).
I know this is not the direct answer you ask for (merely how to strengthen your back with a pullup bar) but reading your question, I think that the solution to your upper back contractures requires an richer approach than the strength exercises alone. Dynamic stretches are a very important part of whatever you do with this kind of situation, and specially correcting your posture and habits during the whole day, not only at the gym.