how do you tell the difference between a worn diff and worn wheel bearings or drive shafts?

I have a noise from the rear - appears to be from passenger side. It increases with load i.e. no noise when I am decelerating.

The noise appears to get worse as the car warms up - i.e. after 10 miles it's definately worse.

Also, .... is it feasible to swop driveshafts over left to right hand sides of the car to get another x thousand miles out of them - I'm thinking the shafts will be operating on the other bearing side.
Steve