From my experience, sore back usually means bike is too big.
For sore knees, there are models called 'semi-recumbents' (example of one here http://bicycleuniverse.info/eqp/#semirecumbents) that are marketed as being easier on the knees. Ditto for some cruisers. Might be an option if you can't find an upright that fits.

Usually, a mountain bike (or hybrid) that has been adjusted for upright riding position can combat both the problems, spend the extra time trying some different models and don't be afraid to modify - a simple stem change or re-position, as examples, can make a big difference in the size and fit.