Originally Posted by
rdtompki
This makes absolutely no sense. The medium cage RD probably won't handle the 34t. Either find a 9 speed Shimano or SRAM cassette with a 28t max. cog or change the RD. It will cost you a lot from money to go 10 speed for very little gain.
I'll bet OP either misunderstood or mis-used "shifter" when, like you say, it's the derailleur's fault.
OP: for $25 - $50, you can buy a Shimano Deore mountain bike rear derailleur that will work with the cassette you have and the shifters you have. Just make sure the rear derailleur is either 8 speed or 9 speed, not the 10 speed versions.
Or like rdtompki suggested, get a 9 speed Sram or Shimano cassette with a large sprocket somewhere between 26 and 28 teeth. I'll bet your rear derailleur is "rated" for a maximum of 26 or 27 teeth, but 28 and sometimes even 30 have been known to work. I have a 28t cassette on mine and it works fine.