First gear

My 2000 323 5 speed, no leaks, bought about 1 month ago at a used car lot, 75K miles, will start vibrating at the shift lever and sometimes can feel in rear end when accelerating past about 3000 rpm in first gear (which I hardly ever do, but tried it after a couple weeks of babying it). Sometimes does it a bit in 2nd but then will smooth out at higher revs. No problems in higher gears. Everything seems tight in the drive train and runs very smooth, even in low rpms in higher gears. Any ideas?