[-c-]

well the guibo or whatever they call it may be seperating, its the hard rubber part that connects the drive shaft to the trans, when they start going you get a vibration as your car builds on speed on hard acceleration they start to deform and the drive shaft starts to go out of balance this will cause vibration and it only does this under hard acceleration, you cant see it unless you pull the exhast and heat sheild off, so its not easy to diagnose, had a simular problem in the 3 series, found out to be the rubber guibo, still not sure of the name. as far as gear goes this also may still be the cause as the torqe from second gear may be enufe to cause it to start pulling apart and the other gears may not have the torqe but then again im just guessing.

id try this before trying bigger fish like something in the tranny