i'm having vibrations and it's pissing me off, i need to figure out whats going on, a little history first, vibration did not exist when 160k miles on clutch/engine/tranny, pulled tranny for a clutch change, installed new flywheel/clutch/pilot bearing rebuilt transmission, new bearings, new slider gear and synchros, new u-joints installed on driveshaft, now i have a vibration at 60-80mph is the worst faster and its still there but not as bad, sounds like a bent fan blade, push in the clutch and it goes away, if you vary the throttle on and off while vibrating it seems to have an affect on it, so i pulled the driveshaft out and installed a different shaft same thing, things i tried/checked.

rotate driveshaft 180 degrees
checked transmission mount
checked ext housing bushing, very very little side to side movement with yoke installed
rear was just re done with new axles/posi/gears/bearings, vibration existed before and after

there is no vibration through the rpms while in nuetral, you can redline 1st, no vibrations

i dont get it....

it's a 1986 305 / T5 /3.73 combo with steel driveshaft.

 

It has to be be something with the tranny.