For the past year i have been trying to figure out why my car vibrates in every gear. the higher the rpm of driveshaft the worse the vibration. it is not the engine,cluch..etc....because the vibration is there when coasting at 80+mph with the transmission in neurtal, and engine at idle. i have replaced all parts from the bellhousing back to no avail (list below) there is no appearant rubbing of parts, but could this virbration be the result or using polyurethane motor mounts, trans mounts???

installed parts:
3 different sets of wheels
HP motorsports TQ arm pinion angle set at -2
adjustable panhard bar..centered axle
swaped stock drive shaft from my 96 Z
new ls1 drive shaft for 6sp cars
changed axles with known good rear end
replaced entire rear wnd with known good rear end
reinstalled orginial rear end...new bearing, motive ring pinion..etc.
replaced transmission built by t56rebuilds.com (this only reduced the severity of the vibration

 

Must be the poly tranny mount... I hated mine. I replaced it with a stock rubber LS1 mount and loved it Haven't been able to break it either...

 

the trans mount could be the problem, i intalled it before i stated dring the car, but ive had the car up to 130mph, 15 or so dyno runs, and 65-75mph driving every time i drive, due to location. throughout the 9300miles ive put on since reubuilding the car nothing has come apart, leaked at all. at this point i am stumped, there is nothing more to replace or adjust...

 

i have the same problem and i have tryed both poly and rubber nothing changes and my will some times be so nasty and othere time its totally fine?????

 

might I suggest you inspect your tail housing bushing and if no problem there remove the tail and check the rear case bushing it should have very little play and you can see it though the back of trans, And also I once had a similar problem with a customers 98 camaro , he had bought it new and the trans had never been out he had been to several shops and replaced the driveshaft, u joints , mounts , etc and nothing but upon further in spection we found the case bushing wasnt Bad It wasnt there at all ! so we replaced it and all was fine