1998 V70 AWD Turbo 162,000

Today I'm driving the car and I start to hear/feel a slight vibration coming from the rear of the car. It got pretty bad on the highway. I pulled over and felt each rear wheel to see if maybe a wheel bearing, no difference in temp. Here's what I notice the most, the car feels like it is shuddering and will minorly calm down on acceleration. Coming to a stop there is a very distinct click click click coming from the back. Cornering does not seem to make any difference. Hard acceleration doesn't invoke any real problems either. Shifitng to neutral make no diff. Engine runs smooth all the time, no lights.
At highway speeds there's a vibration being felt. It is the slowing down that brings out the worst of the sound.

Does this sound like the center prop-shaft getting ready to go? Rear differential?
All advice greatly appreciated. !62,000 miles on the car. Good tires, etc. Definitly a mechanical sound from the rear of the car.
Any and all advice very appreciated. Thanks