a vibration problem at 40 mph

[mach0415]

I would also check the u-joint. FWIW A u-joint will vibrate at those higher speeds under power and then when power is removed in a coast down, vibration will decrease.

 

[George Miller]

I think George means the one inside the front of the torque tube !!
John Cochran

Yes that is the one, very important. When the drive shaft is turning in the 2000 rpm range, it will let the U joint move around on the back side, if it is not there.

 

[columbiA]

A friends car had a bad vibration over 35 mph.The problem was that the inner ball cap around U-joint was not installed.Front of TTwas free to jump up & down & DS moved forward enough to knock out the brg stop on trans case & required another case to repair.