I was gonna say mounts, but maybe you want to inspect them to make sure they're secure.
I've seen where engine mount extension has broken, also where the transmission mount bracket has broken.
I also have M3 engine and motor mounts. I do get slightly more vibration, but only between 800-1000 rpm. I have a stock shifter though.
It could still be the guibo... just that certain vibrations with more load in first gear maybe? kind of a stretch, but worth looking if the guibo has never been replaced.
I'm at 180k and my guibo is cracked. i have a new one ready to go in soon.
What about on heavy high rpm engine braking in 1st gear?