If the shifter feels sloppy, there's a bushing at the bottom that gets worn out. It a plastic part, and some places sell "upgraded" metal ones.
https://www.nissanparts.cc/oem-parts...ore-32861h7301
If it's the whole transmission shaking at higher RPMs, I replaced the trans mount with a Nismo mount and it made a world of difference. I'm sure Nismo engine mounts would also help tighten things up, but so far just the trans mount helps a ton.