Unless you're set on buying a new crank, I'd try what you have with a 11-34 cassette enabled with an XT or XTR derailleur. These will work fine with STI shifters and allow you to still use a road cassette when not touring. If you think you need still lower gearing, you can get a Sugino XD600 which gives you 48-36-26. Any smaller chain ring than 26 could create a frame clearance issue.

Play with Sheldon Brown's gear calculator to see what might work for you.

http://www.sheldonbrown.com/gears/