I'm a little confused. Wouldn't a shorter stem would put you in a more upright position by moving the handlebars closer to the saddle. It seems to me that you might be able to mitigate the problem either be going more upright and getting more of your weight on the sit bones (were any of those saddles you tried made for a more upright position?) You might also be able to get more comfortable with a less upright position as with 531phile's suggestions above, though the geometry of the Frontier does not look as if it would work well with that. Good luck.