Am I going to need a step up transformer?

Just sold me beloved Benz Ruby 3 that was rated .34 mv output and now thinking about a Dynavector XX2 MkII rated at .28 mv. My phono amp has a gain of 70 db. I would say I had my volume control on my pre amp at about 2:00 for loud listening with the Benz. I really want this to work without a SUT.
After your 70 db phono gain you will be down about .12 volts. It just means you will have to turn the volume up a bit more. Usually, in a pre amp, noise floor is what you will be dealing with as you turn it up a little more. I suggest you turn it up to your normal "max" level without any music playing and then see how much the noise increases as you turn it up. I'm going to guess that you will be about 3 o'clock to get the same "max" volume.

BTW IME you need to more concerned with the recommended load impedence of the new cartridge, not the output voltage. Can you change the input impedence of your MC inputs. My Dynavector's sound stage is dramatically better when I got the "load" down to 30 ohms.
You should be fine. From the KAB gain calculator web page:
Chart of Typical Gain Values (relative to 5cm/sec):
Output Gain
3 - 6 mV 36dB
1 - 3 mV 43dB
0.3 - 1.0 mV 53dB
0.15 - 0.3 mV 60DB

Of course, YMMV. For more details, click here.
The difference between .28 and .34 is less than 2dB so you should be fine.

The closer you are to full volume the better your system gain is set up. Excess attenuation (hair trigger volume controls) just means you are amplifying (adding distortion) just to attenuate (add losses).

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Although there is no standard method for measuring output, so the two numbers may not be an "apples to apples" comparison, the difference appears to be quite small. Also, 70 db is quite a lot of gain for a phonostage. You should have plenty of gain for that cartridge. A friend uses it and really likes the Dynavector.