RE: Drive shaft Dampener????
If I remember right, you guys have the steel shaft. Due to the length, about 70",steel is not as forgiving as the aluminum and will flex causing a vibration or failure. Ask you driveshaft guy if he can determine the 1/2 critical and critical speed of the shaft and that may answer your question as to the dampner. If it's approaching the1/2 critical speed of the shaft, the dampner my be offsetting the effects. Then again, it sounds like your particular shaft may not need it if it does not vibrate w/o it.
Shaft speed is final engine rpm's / final drive gear. Critical speed will be determined by length, thickness, diameter,type of material and other factors.
In my case 2500 rpm's(at 70 mph)/ .67 (final drive gear) = 3731 rpm's on the shaft at 70 mph. This is not accounting for torque converter slippage.
I was thinking of having a new shaft made for my truck. It will run about $325 for a 4 3/4" x 66"diameter shaft made from thicked walled aluminum - can't remember the thickness off the top of my head.