I can do my best to help with this but I've only been working at a transmission shop for a few months.
Believe it or not the issue most likely isn't even with the transmission itself. Everything inside a transmission is inherently round so it almost never vibrates. The only times you will see a transmission vibrate is if you're missing teeth or if you have a bent mainshaft. Both of which you will have more obvious symptoms. The most common causes of vibration issues with our transmissions are a bad pilot bushing or a clutch housing that's out of line. Seeing how this issue is only in one gear the problem could be the shifter as well. Definitely replace your transmission fluid. And DONT overfill it.
Sorry, that's all I got. If you're still having issues after you replace the fluid send me a DM I know some manual transmission gurus at work I can ask.
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"'He who shall be last, shall be sideways and smiling.' - Baby Jesus" - Jeremy Clarkson __________________
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