5th gear vibration/ shudder

I have a 4.6 dohc with a tremec 3650 trans. So at 1850 to 1950 rpm in 5th gear at around 59 mph I feel a intermittent vibration/shudder. while keeping the rpm constant at 1900 rpm this vibration goes on and off in approximate intervals of 2 seconds apart. it comes and goes very quickly. This does not happen in any other gear at 1900 rpm. I'm leaning towards driveshaft?? any input would be appreciated.

 

More likely it's in the transmission. The shaft has no moving parts other than the U joints

 

if the operating angles of the u-joints are off, it will create a hamoninc vibration like you described.
you need to check the pinion angle in relation to the motor trans angle. also need to check that the motor and trans are straight in the car and that the rear end is straight and square in the car.

 

Just to help me understand. Why wouldn't this vibration that comes and goes in 2 second intervals be felt in all gears at the same rpm?

could an unbalanced drive shaft cause the vibration?

and thanks for the input

 

ive never had an unbalanced drive shaft act like that. i have had bad operating angles do that.
read this and see if it helps
http://www.spicerparts.com/measuring.asp

 

Thanks for that article.

On a level surface wouldn't I be able to determine the angle difference on the driveshaft from taking floor to driveshaft measurements ... 1 near the trans and the other near the rear end.. or I'm I barking up the wrong tree?

 

barking up the wrong tree.
you need the centerline that runs through crank shaft and trans to be pointing streight down the center of the car. the fact that you have put a 4.6 and a 3650 trans means that you have done significant fabrication to your car and there is a good chance that the centerline is pointing to the left or right side of the car.
if it is not then you can work on the angles of the rear end.
it needs to square with the center line also. if its crooked as in its pinion shaft pointing to the left or right then that will need to be corrected.
after every thing is square in the car you then set pinion angle.
you just use an angle finder to determine what angle the motor and trans centerline is set at. it usually has a downward angle to it. so the pinion angle will need to be two degrees down from what the engine angle is.

 

Thank you! ... I understand what needs to be done.