That's a good point on the RUBBER in the balancer, it's more about time than miles with rubber I guess. They used to measure ozone exposure using cracks in a rubber band as a gauge, ozone attacks rubber. Also, I had one go out on a road trip in my 60-series and that made for quite an...
Definitely +1 on NOT pulling the engine. I assume you haven't dropped the tranny in these because that's the only reason why you'd think pulling the engine would make this "easier". =)
Harmonic balancer failures are rare enough, and your mileage low enough, that I'm not sure there's a good...