Thanks for the kind response
Have been through a lot. The driveshaft is a Single Piece Steel driveshaft taken from a 1998 LS1 Trans Am (the previous owner (t/a) upgraded to Aluminum).
It is most definitely SPEED based - not RPM based. I can rev from 0-5000 rpm in 1st gear (or even in neutral) and get no vibrations whatsoever. However on a dyno, on the street, wherever I am driving, if i hit 3000 rpm in 5th gear (80+ mph), I feel a HUGE amount of vibrations..feels almost like the car is going to fall apart (I try very hard not to reach this speed ever)
Hope this helps to help me and thanks everyone