Aoa,
I recently did an engine swap in my RX80 Cressida and along with it changed all the suspension bushes. Got a used rear differential assembly and also had to modify the front half of the driveshaft to accommodate the gearbox.
Since this conversion I'am getting a high frequency vibration that starts at 30 km/h and is coupled with a low frequency sound. This vibration increases as speed increases. It can be felt in the whole body. When the car reaches 100 km/h another low frequency vibration comes up that is similar to all 4 wheels being way out of balance.
In trying to reduce this vibration, I have taken the car to a number of driveshaft "specialists" who say they have the proper balancing equipment but in reality have a lathe machine and their eye. The axle's in the differential have been changed but the vibration is still there. I think that the driveshaft is the main problem as the front half was cut short and welded back together. @Xulfiqar bhai would you mind shedding some of your knowledge on this.