Someone solid fill their rear engine mounts and see if it solves the problem. DO IT, DO ITTT!!
I've been in a number of cars with slight grinding problems, shifting problems. I had a FWD Turbo Pulsar pumping 161.8FWKW and axle tramp/torque steer was it's best friend. I managed to slap in any gear no matter where I was bouncing on the road after I had installed my solid rear mount :D Also, a 180sx, s15 and 350z - friends and I had the engine dampening kits on and it solved almost all problems with mis-shifting in high RPM.
For the people without stuffed syncros as of yet, it's a good prevention method.