I added a new pull up section http://www.simplefit.org/pullup.html
I would say your best bet if you want to get as close to a pull up as possible would be with body rows. You can vary the resistance by changing the angle of your body. They are really great exercises for developing the pull up.
Here is a great article on body rows
Rings are great to have and easy to make I will try to find an article.
You can go up a level or keep working this one focusing on getting you pull ups, your call.