For the last 30K miles there has been a whinning noise coming from my truck’s rear end. The sound is getting louder and typicaly occurs from 20-60 mph. Not dependent on engine rpm. The noise will get a bit louder if I coast. The noise will go away if I go around a curve to the right but will be constant on a left curve or straight ahead. Changed differential fluid with no change. The trunk drives fine otherwise.
I would start looking for a used replacement rear axle, the entire assembly, brakes and all if you can find one. That way you will be ready to go when the time comes. Try to find the same gear ratio as you have now. There will be a tag on the differential housing or cover that lists the gear ratio, something like 3.23, 3.55, 3.73. If your truck is 4wd, this is a MUST…
I agree; it may be time for another rear axle because the ring and pinion gear mesh is off on this one.
Either that or you just have a bad wheel bearing on the passenger’s side. Jack it up and spin those tires.
This is what I would check first as well. Hand rotation might not be definitive if you catch them early on. And also be sure it is the rear end. Bearing noise is sometimes very hard to nail down which end is causing the problem. Assuming it’s in the early stages of going. I even rode in the rear and front to try to isolate the noise. Of course, when they really start to roar, it’s not so hard to tell 
I recently had a very hard time telling if the rear bearing was going versus the front. Fronts wear more often and so I took a chance and that was it.
Edit- The OP should at least check the fluid level in the rear end to be sure it’s properly lubricated.