5ktq massive vibration from rear 20-40mph

[Kurt Deschler]

I am trying to isolate the source of massive vibration from the rear of my 
5ktq. The problem is definitely dependant of wheel speed, not engine speed 
and comes on hard and fast at 20mph. Problem also occurs with the rear end 
up on jacks. I tried adjusting the prop shaft bearing and geasing the 
U-Joint and neither helped. I have also ruled out the brakes. Could the 
rear diff be bad? It used to weep fluid so it is possible it went dry. The 
vibration reasonates the entire back of the car and is enough to rattle 
your teeth out.

 	-Kurt