I have had issues with Shimano derailleurs taking a 34T cog, although it is mostly the early 1990's models with this problem. The first adjustment is to run the B screw (at the top of the derailleur behind the mounting bolt) all the way in. On early 1990's Deore DX derailleurs, this still is not enough. On those, as well as some newer Ultegra 6500/6600 SGS models, I take the B screw out, and run it in from the back side (the head of the screw rests on the drop out). I have been running two bikes with 34T cogs this way for years without any issues.