Sorry to hear about your back. I'm not sure why needing an upright position would necessarily preclude a road bike, however?
Have you considered converting your road bike to a flat bar setup and/or raising the bar height with stems and spacers? You could easily end up with a seat-to-bar drop that is similar to ATN's bike (Banshee), which is set up in the photo for XC riding. You could even add cyclocross style bar top levers for easier brake access on a drop bar.
Also, you might also try posting on mtbr.com, which tends to be more XC-oriented.
Good luck.
"The song of a bird…We used to ask Ennesson to do bird calls. He could do them. How he could do them, and when he perished, along with him went all those birds…"-Return from the Stars, Stanislaw Lem
"We just walk around, and sometimes we go out and dance, and then we listen to the environment."-Ralf Hutter, Kraftwerk