I have a 1980 Rx7. Manual tranny 5 speed. At higher rpm (~4000) in 5th gear there is a vibration which I believe may be coming from transmission. When it drops to lower rpm no vibration. In all other gears any rpm no vibration. U joints on drive shaft appear secure New tires, balanced wheels, front end aligned.
Does anyone have any thoughts or recommendations? Is it a problem with 5th gear in transmission or possibly something else? Thank you.

 

Tranny mounts maybe? Other than that I would sah something with the driveshaft. Have you had the balance checked on the driveshaft? What condition are the motor mounts in? Clutch and pressure plate? Ok well the last 2 are kinda stretching it but they are in the affected area and they rotate..

 

I had the same vibration. It was a Tranny Mount. Smooth as silk now

 

i would look into pulling and inspecting the trans mount for cracking or for separation of the rubber isolator from the metal frame mount.