I removed the halfshafts & driveshaft today to replace all the unis, upon completion I connected only the driveshaft back to test for any vibrations (the halfshatfs are not connected), so the diff companion flanges just spin by themselves. It was all smooth until about 80kph or 50mph then I can hear the tunnel vibrates, kinda loud. If I rev it pass that speed then its fine, tried gear 3,4 and 5 and all have the same problem.

When I install the unis I put both the grease nipples on the same side (the nipples are on the caps) instead of opposite sides, could this be the culprit to the vibration? They are a few grams max, could it be that they caused the vibration since we are talking high rpm here? Ive yet to rotate the driveshaft 180 degree and I'll do it tomorrow, also since they tunnel vibrate without the power transmitted to the ground Im guessing the same thing will happen when the halfshafts are connected?...Please give me some inputs, I dont really want to remove the one uni and rotate the nipple to the other side. Thanks

More likely the imbalance is more than the zerk fittitngs. I'd guess that the front yolk is installed on the rear shaft yolk or one of the yolks is 180 degress out of phase. Easiest fix is take it to a driveline shop and have the shaft rebalanced. Also check your diff mount and the mustache bar bushings as well as those bushings on the back of the transmission. My 240z had shot tranny bushings which settled things down a bunch after replacement.

Ive marked the yolks so thats not an issue. But Ill try the 180 degree rotation and rotate the grease nipple 180' if that doesn't work.

Ive marked the yolks so thats not an issue. But Ill try the 180 degree rotation and rotate the grease nipple 180' if that doesn't work.

You got a nice test lab setup with your car to try all 8 combinations, but eventually one of the C clips in the U Joints will bend.

Remove the grease nipples and see how it goes.

I removed the halfshafts & driveshaft today to replace all the unis, upon completion I connected only the driveshaft back to test for any vibrations (the halfshatfs are not connected), so the diff companion flanges just spin by themselves. It was all smooth until about 80kph or 50mph then I can hear the tunnel vibrates, kinda loud. If I rev it pass that speed then its fine, tried gear 3,4 and 5 and all have the same problem.

When I install the unis I put both the grease nipples on the same side (the nipples are on the caps) instead of opposite sides, could this be the culprit to the vibration? They are a few grams max, could it be that they caused the vibration since we are talking high rpm here? Ive yet to rotate the driveshaft 180 degree and I'll do it tomorrow, also since they tunnel vibrate without the power transmitted to the ground Im guessing the same thing will happen when the halfshafts are connected?...Please give me some inputs, I dont really want to remove the one uni and rotate the nipple to the other side. Thanks

A little too late now, I realize, but did you try this before replacing the U-Joints? If not, for all you know this problem, if it is indeed a problem is preexisting and located elsewhere.

Are you sure there even really is a problem? Any system of rotating parts will have a harmonic at some point and having one at 50mph under a condition unlikely to be experienced, that is with no half-shafts, is as good as place as any and better than many to have it.

Chris