I found the below test on a driveshaft site. I thought it had some good info.

DRIVESHAFT'S
Most people have trouble determining if the vibration in there vehicle is coming from the DRIVESHAFT or not.
There are typically two type's of vibration most vehicle's would have.A fast cycle vibration or a slow cycle vibration. To help understand this lets say we put a bucket of water on the passengers floor. If the vibration in the vehicle produces small ripples on the top of the water this would be considered a fast cycle vibration. This type of vibration is usually a drive train vibration,things like the driveshaft,motor or torque converter. If the vibration kind of puts waves on the top of the water. This type of vibration is a slow cycle vibration and usually is a axle or tire vibration.

JACK TEST
A simple way to do this is put the
vehicle in question up on jack stands (make
sure the vehicle is completely secure)block
the front tires and run the vehicle up to the
speed you have the vibration at. Make sure you
use the brake to stop the drive train before
you put the car in park if its a automatic.
If the vibration is a fast cycle vibration
you may want to have the DRIVESHAFT checked
for balance or This may make no sense to you but
you may try indexing the shaft 180 degrees(just
pull the shaft off the rear yoke and put it on
the opposite way)what this does is change the
resonant property of the driveline and in many
cases it takes the vibration away.
If you have a slow cycle vibration take
the tires off the car (make sure you put lugs
on the axle to keep the brake in line) and run
the vehicle again. If the vibration is gone you
now have to find out if its the rim or the tire
and good tire shop can help you with that.