Now what???

[crawdad]

Man, if it ain't one thing, it's another. Now that I've got my new exhaust on the truck and everything is a little quieter, I've discovered a roaring coming from the back of the truck. It's almost like a low pitched roaring/grinding sound and it is definitely RPM related. I hadn't noticed it before, but I can sure hear it now. My first thought would be a bad rear axle bearing. What do you think, and how could I check this out to be sure before I go tearing into my rearend?
One other thing that has reared it's ugly head is a popping or a metallic thunk when I go from park to drive or from neutral to drive. It ain't so bad from park to reverse. Is this u-joint problems you think? Thanks for the help.