I've been getting a clicking noise from my car at high speeds when it's engaged in gear.

It would start up after maybe forty minutes of driving on the highway around fifty five or sixty miles per hour. Sometimes it'll get louder, then it'll back off, then start again. If I left the car in gear and let it coast down a hill, the sound would also stop. This leaves me to believe that it is not a transmission problem, also the sound seems to be coming from farther back on the car. I've inspected the flex discs, both seem to be in great shape. Also, the transmission shifts very smoothly and there doesn't seem to be a problem with it at any other time.

This leads me to believe that there is something wrong with the rear axle. Does this sound like a correct hypothesis? Any ideas as to what part of the rear axle are prone to failure?

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1983 Stick Shift 240D (223K)