Worn-out U-joints. Or if you are slightly mis-diagnosing it, then you need to re-balance the unit. First check your U-joints and the condition they are in first. Also, how many miles on the car?
more than likely it is the u joints unless you lube them often they go bad! new ones are fairly cheap @ napa or ford. If that doesen't fix your problem then fms makes a aluiminum one for around 178.00 you can pick one up @ Summitt! Good luck