Low RPM Vibrations?

[IbanezRG]

Hi guys, I just purchased a 06 G35 5AT Coupe about 6 months ago with 60k miles. It's been a dream to drive so far, however last week I started feeling the entire car vibrate/shake when at low RPMs on gears 3-4.
So lets say I'm driving at 40mph or so, and release the accelerator and let the car cruise. When I re-engage the accelerator lightly it starts at a low RPM and vibrates quite a bit.. only when I press the accelerator harder does it drop down to a lower gear, and the vibration is gone with high rpm.
Really hope my transmission isn't failing. I barely have any money to get this checked out at the shop, I most definitely can't afford repairs!
Any help is much appreciated.
Thanks!

[audtatious]

Do you know if the transmission has ever been flushed? That may be the first thing to do if it's not had one and they can do some cursory investigation when doing it. I assume you can recreate this at will? If so then you can at least go by the Infiniti dealership and see if you can get one of their mechanics to take a ride with you.