Several options:

* IRD http://www.interlocracing.com/freewheelbreakdown.html has 6 speed freewheels with 32 and 34 tooth big cogs. Yes, you will likely need a new RD.

* Like this one: http://www.amazon.com/Shimano-Tourne...2265037&sr=1-1
remember on the Sovereign; you need a derailleur with a hanger, or purchase a seperate hanger and higher end RD.

* Megarange: http://www.amazon.com/Shimano-MF-TZ3...2264866&sr=1-2 I do NOT like the jump from the biggest to 2nd largest cog - murder on ones knees. Basically, they just replaced the 28T with a 34T cog. IRD has fairly evenly spaced ratios.

* Compact double crankset - I suggested this to you earlier, gives you 50-34 in front instead of 52-40 (or similar); reduces wieght, lowers the gearing, and then a IRD 13-24 works well. The 50/13 combination is taller (faster) than the 52/14 combination, and the 34/24 is fractionally lower than 40/28; and your present derailleur works. You should probably shorten the chain a few links. And if you need lower gearing, the IRD 13-28...