What is a carrier bearing?
These are the bearings that support the inboard ends of the axle shafts. They are located inside the differential. I wouldn't worry about them yet. You have lots of easy stuff to check before you head down that road.
It is also another name for a center support for the driveshaft - but I doubt you have one.
ATF level is fine.
That's good.
I don't know if idle is high - don't have a working tach yet.
If you don't have a tach, that's okay. You can still reduce the idle speed to determine whether that is contributing to the problem. Just back the idle speed screw out a turn or two. Do it in small increments until the engine is revving slower but not near the point where it would stall. You can always crank it back up if you get no improvement - just count the number of turns that you turned the screw out.
You need to check:
1. U-joints (far and away the most common cause of drive train "clunk" condition).
2. Transmission mount (also check your engine mounts)
3. Axle u-bolts and perches
The best way to do an initial inspection for the source of a drive train clunk is to simply lie on the ground as close to the truck as you feel comfortable with. While an assistant slowly shifts the transmission from R to D and back (over and over), you will be watching the various components and trying to identify which one is generating the noise.
Sometimes the culprit can be easily identified by a jumping or jerking motion, other times the source is obvious simply by the direction of the clank sound.
That first step might not exactly pinpoint the problem but it may give you a good idea of where to start.
After that, get under the truck with a long pry bar, a pipe wrench, a hammer, etc and try to locate loose components by applying force to them.
A hint on u-joints:
A seriously worn u-joint often manifests itself by causing the truck to shake when the load is suddenly dropped while at highway speeds.
