3rd gear at least on my car is 1:1 so drive shaft RPM = engine RPMs regardless of your rear end gear. My understanding is that the reason the police driveshaft is on the CVPI in the first place is because it is shorter than the stock CV GM driveshaft just to raise the frequency of the first bending mode. The tailshaft is longer on the tranny to make up the difference. Critical shaft speed is really not the reason for problems (I am sure it is much higher than the engine can spin it) the first bending mode would have the largest deflection ( and might have a frequency you could feel.)
I have at my disposal FEA software that I could use to get the natural frequencies of the 3 driveshafts in question here. Al I need is ID, OD, length, and material.
Anybody have it or know where I could get this information and at what RPM do you first feel the vibration at?
2003 300A "Morticia"
2009 Expedition
1996 E250
FASTEST SPEED POST: 25WPM at 78MPH
(can only improve with practice)