I just noticed about a week ago that I was hearing a low howl, almost a whine, from what seems to be the rear end. I can only hear it going between ~42-45mph, and when i'm basically coasting in that range. If I accelerate or decelerate right through it, I don't hear it at all. It seems like it started to happen when I let it sit for a week without driving it anywhere.

Rear diff fluid was change about 1k miles ago, and there was nothing out of the ordinary when I changed it. I pulled the diff fill plug the other day and the fluid level is right at the bottom of the fill hole, and looks brand new.
My rear DS was replaced, under warranty, about 5k miles ago and the u-joints look good.
I was going to check the carrier bearing, but just wondering how I can tell if it's worn? Any other ideas on what to check?

Oh, and it's an '06 2500 crew cab, long bed with the G56.

 

If you're referring to the hanger bearing on the driveshaft, it won't likely have any visual sign that it's noisy, unless it's REALLY noisy. Two ways to check it are to partially remove the driveshaft from the truck so you can give the bearing a good spin and feel for roughness, or you can jack all the wheels off the ground, support the truck with safety stands, have a helper run the truck in gear while you listen to the bearing with a stethoscope.

 

You may want to check your fluid level again in your rear diff.
The fluid level is actually supposed to be about 3/4 inch below the fill plug. There is a marking on the diff cover. I believe if you fill it to the bottom of the plug, you could get foaming of the fluid. Depending on the type and viscosity of the fluid, I believe it could make your gears whine.