Rear end noise: CV axle problem?

I have a 1983 300CDT (212K miles) that I purchased in May 2007. It drove solidly until about 4 weeks ago when driving up long hill around 50mph I heard a rhythmic clunking sound coming from the right side of the rear end. The noise disappeared as I took my foot off the accelerator. After coasting down the other side of the hill, the sound didn't come back, even under power, for the rest of the drive home (about 20 miles), or in 2 weeks of daily commuting afterwards.

The problem returned again on a recent weekend trip that involved a long hill (4 miles at 10% grade). About halfway up, I heard the clunking again. I pulled over to see if I ran over something and to check the transmission fluid level (fine). Started driving again and the noise went away for the rest of my drive (about 1 hour). The next day, driving home, the noise came back again, also about halfway up a long hill. It didn't make the noise while coasting, only when under power. Unfortunately I had to drive it another 20 miles to get out of the boonies. By the time I got out of the mountains and into cell phone range to call AAA, the noise was load and apparent when coasting. Got a tow home and when I had to pull it in the driveway, the noise had disappeared again.

I took it to my mechanic who could not replicate the problem, but feels (based on his experience) the problem lies in the CV axles and replacing both CV axles ($800 for parts/labor) is necessary. He found a torn right rear CV boot as well, which would be from the area the noise was coming from. The flex disk looks fine and the transmission is shifting solidly.

Has anyone had a similar experience and/or suggestions? Do CV axles have to be replaced in pairs? I bought it from its second owner who had most of the service records, but nothing recent about the transmission/rear end.