My '84 has made a humming at highway speeds since 300K. It now has 430K on it and while the noise has not gotten better, it has not gotten worse, either. This has been going on for 8-9 years at this point. The Dana 30's in our cars are pretty hearty units and given the relatively low horsepower and torque that our cars make (compared to other makes/models that the Dana 30 has been used in), they tend to wear very slowly.

The clicking sounds like a u-joint. The rear u-joint is a somewhat common failure. You might want to check your center carrier bearing as well.

Personally, if it were me, I would switch the rear end to a synthetic lube and not worry about until if/when it becomes a bigger problem. That could be years from now or never. I'd also check that rear u-joint and see if that isn't the issue.