Drive shaft balance
 

Imagine that I have a imbalanced drive shaft, at which speeds should I notice that?? I had a decayed transmission mount, after replacing this the harmonic vibration was deduce a lot, but when reach speeds above 55mph an other vibration seems to enter...

did you check the u-joints? unless a weight comes off the driveshaft.. they dont just go out of balance..

check the wheel balance too...

Try running your transmission in two different gears at the speed you get the vibration.

If it vibrates in both gears at the same speed its in your tires.

If it does not vibrate at the same speed, its in the driveline.

Confusing but I hope this narrows it for ya.

No, but they do get bent and bent shafts vibrate. A simple check is to remove the driveshaft and roll along a flat surface (table) and see if the middle bows or rolls flat. If you can see daylight under the driveshaft and it gets bigger/smaller as the shaft rolls chances are it needs work.

Almost any light duty axle shop can balance and check "u" joints for a small fee.