Several people had their torque converter replaced under warranty. I hadn't heard of it on a 2006 model, I thought it was just the 2007 to early 2009 models. I have a July 2009 manufacture vehicle and it doesn't do it.
As far as I know it's caused by the TCC not engaging correctly and then disengaging, then making the attempt again - in quick enough succession to cause a noticeable vibration. There's no cure apart from replacing the torque converter that I've heard of, although I do imagine that with a few faulty units you'd be able to figure out what is wrong and possibly do something to it - but all the faulty units that I know of are with Nissan who MIGHT know what it is but getting an answer from them wouldn't be easy.
Now, all that said - it might be worth your while going to your Nissan dealer and asking if they could approach Nissan to have it done as a goodwill gesture. You might suggest covering the labour costs (should be about 3 hours, if memory serves) if they're reluctant. Nissan have known about it for some time and might come to the party.
Good luck!
