vibration

I was driving down the highway today and all of a sudden when i was doing about 70 i started to feel a bad vibration in my wagon that appears to me to be coming from the rear of the car. so i let of the gas and it went away for a lil bit then would come back, and then let off the gas again then after a lil time came back again. so on my way home from work i stopped at my to my mechanics shop to ask him what he thought. he said it could be the universals, rubber things on each side of the viscous coupler, or the coupler it self. the car is a 1991 RT AWD 6spd. just looking for some opinions from you all to see where i should go from here. thanks