Will a new tranny mount cure my problem??

Does the shifter vibrate at high rpms when in neutral? Or only when in gear? Or all the time? I would think that a bad mount would exacerbate a bad vibration, but not be a cause for one. If your mount is bad, changing it would reduce the vibes, but I don't think it will cure it. I wouldn't rule out the driveshaft tho, since it seems the variables that you figure could never cause the problem seem to be the problem in the first place. (murphy's law)

I'd be surprised if the mount fixes it unless yours is broke. Because my buddys 86 GT [Whitescones on PerformanceShop BB] has 3.73s [we jus put em in - car will be mid 13s now..] and the stock 260000 KM orig tranny mount and no real vibes at all up to 5300 RPM really... He's getting a urethane one anyways cause his is really worn....

What about your crossmember??? Hows the rubber bushing where the mounting bolts go through? Driveshaft bolts... you used blue loctite? Years ago a friend with an '80 Capri RS Turbo, that he put a carbed 5.0 5 speed in, came by with a bad vibration problem [shifter/ car / everything vibed] and he had 2 bolts left holding in his driveshaft. I remember he said it started out as a small vibration in the shifter and just kept getting worse till it sounded like all hell was breaking loose.. I looked under his car and right away I saw the prob and I --> His D/S bolts just loosened and worked their way out to the point where there were only 2 left there, threaded half way out the pinion face ready to fall out

Motor mounts in ok shape? Mine had 160000 KM on them and they were cracked pretty bad but I never got vibrattions in the shifter from that. I'd like to think its something from the tranny back, except... maybe... harmonic balancer because that could affect engine AND trans as far as vibration. Have a buddy turn the car over [coil unplugged] and watch the balancer to see if it wobbles. See if the rubbers all cracked in it too.

Get under there and check bellhousing to block, and tranny to bellhousing mounting bolts make sure they are tight. Any slop in the pinion yoke where the driveshaft bolts to? Maybe the bearing there is starting to go but you'd think that would whine tho.

Is it the original trans from 1985? How many miles on it? It could just need a rebuild too.. I hope it's a simpler fix than that!!!

Good luck!

one time my water pump was going out and i thought my tranny was gonna blow, changed the water pump and the vibrations went away.