Doesn't make sense unless there is a resonance frequency issue.
Maybe the pressure was different from before the nitrogen was added to the tires. If the tire pressure is different (or if the nitrogen expansion characteristics are different) then there may be a resonance factor in the tire/wheel/steering systems that when it gets at certain speed it amplifies the vibration.
So if that is the case, then just changing the air pressure will fix the problem for those cases that are not caused by defective tires and/or incorrect wheel/tire balancing. In my case, I had defective Michelin tires on my brand new MDX, and after replacing them for brand new tires, balancing them, and aligning the vehicle, I got rid of my problem. 49,000 miles and no vibrations.
When there is a resonance issue, the vibrations don't get cancelled but they accumulate and therefore the shaking increases potentially to very high levels. This will happen only at certain frequencies/speeds.
2011 Pearl White MDX Advanced
... OEM towing, 20 mm wheel adapters,
... rails/crossbars, black wheel well trims,
... side windows acrylic deflectors,
... Michelin Latitude Diamaris HP 285/45R19,
... Full size spare tire,
... 6000 Kelvin HID's
2010 Acura TL Tech.
2001 Mercedes ML 320
2005 Mercedes ML 500 AMG Package